Obstet Gynecol Clin N Am

Prenatal Diagnosis and GeneticScreeningdIntegrationinto Prenatal Care

Valerie J. Rappaport, MDDivision of Maternal Fetal Medicine, Dept of Ob/Gyn, University of New Mexico

School of Medicine, MSC 10 5580, Albuquerque, NM 87131, USA

Prenatal care was originally developed to protect the health of themother. Evaluation of the health and development of the baby has beena more recent purpose of prenatal care. Birth defects, while recognized sinceancient times, were felt to be due to a whim of nature, exposure of themother to adverse events, or perhaps to the inferior character of the motheror father. The process of fetal development was largely hidden from viewand understanding. In the last 3 decades, perinatal medicine has madetremendous advances in scientific knowledge and in the successful applica-tion of this knowledge toward understanding the fetal aspects of pregnancy.These advances include technology that allows us both direct and indirectaccess to the fetal compartment. The rapid transition of new genetic knowl-edge and DNA analysis from bench-top to commercial applications hasgreatly enhanced the ability to diagnosis genetic disorders before birth.Refinement of invasive fetal diagnostic techniques has increased the safetyof these procedures and has opened the possibility of in utero therapy.Ultrasound in particular has revolutionized the field of obstetrics by provid-ing detailed imaging of the developing baby. This ability to view the fetus isnot only important in the medical evaluation of the pregnancy, but has alsobecome one of the central features of the prenatal experience for the preg-nant couple. These advances have brought about a dramatic change inhow the fetus is conceptualized. No longer is the fetus perceived as onlya part of the pregnant woman, but is increasingly perceived as a distinct en-tity that can be the independent focus of diagnostic testing and individualtherapy. Evaluation of the health of the fetus and screening for birth defects

35 (2008) 435–458

E-mail address: vrappaport@salud.unm.edu

0889-8545/08/$ - see front matter � 2008 Elsevier Inc. All rights reserved.

doi:10.1016/j.ogc.2008.05.002 obgyn.theclinics.com

436 RAPPAPORT

has become an important part of prenatal care. All women in the UnitedStates are now offered some form of prenatal diagnosis or genetic screeningduring pregnancy.

Overview of birth defectsdscope of the problem

Any practitioner who engages in the care of pregnant women will also,sooner or later, be involved in the care of a pregnancy complicated by a fetuswith a birth defect or developmental disorder. Congenital abnormalities area frequent occurrence in human reproduction. It is estimated that more than50% of first-trimester spontaneous abortions and 6% to 12% of stillborninfants have chromosomal abnormalities. About 25% to 35% of stillbirthsare associated with intrinsic anomalies at autopsy. These include singlemalformations (40%), multiple malformations (40%), and deformationsor dysplasia (20%) [1]. Approximately 3% of live births are complicatedby a major congenital anomaly or genetic disorder. Half of these anomaliesand disorders are detectable before or at birth. According to estimates,about 7.9% of individuals are diagnosed with a genetic condition or birthdefect by the age of 25.

Causes of congenital anomalies

Congenital anomalies can be categorized into three groups based onetiology: (1) polygenic or multifactorial causes; (2) genetic causes, includingchromosomal and single-gene disorders; and (3) environmental causes,including such external influences as maternal medical disorders andteratogens.

The most common causes of structural birth defects are polygenic or mul-tifactorial. Polygenic disorders are conditions caused by a combination of ge-netic influences as well as a coalescence of other factors, including maternalenvironmental factors. About 65% to 80% of birth defects are classified asmultifactorial. Included in this group aremanyof themost common structuralbirth defects, such as cleft lip, neural tube defects, gastroschisis, and clubfoot.These disorders do not follow classic Mendelian inheritance patterns but mayshow some degree of family clustering. Most commonly they tend to occursporadically without previous family occurrences. The recurrence risk in gen-eral is highest in first-degree relatives, and then rapidly tapers off, so that therisk level reaches population baseline for third-degree relatives.

From 10% to 25% of birth defects are felt to be due to classical geneticabnormalities. This group includes inherited recessive disorders such ascystic fibrosis; X-linked disorders, such as muscular dystrophy; autoso-mal-dominant disorders; and mitochondria inheritance. Genetic abnormal-ities can also occur as a result of alterations in contiguous genes, typicallymicrodeletions. One of the most common microdeletion syndromes isdel22q11 resulting in a variety of clinical outcomes, including DiGeorge

437PRENATAL DIAGNOSIS AND GENETIC SCREENING

syndrome and velo-cardio-facial syndrome. Also in this group are disordersresulting from duplications or deletions of entire chromosomes, such asDown syndrome (trisomy 21) and Turner’s syndrome (monosomy X).Although these disorders are genetic in origin, familial occurrences maynot be present. Many recessive genes remain silent in a family until thechance pairing of a couple with the same recessive gene. In addition, becauseonly one quarter of offspring will be affected, the condition may not showup until there are already many healthy children in the family. Autoso-mal-dominant conditions in the fetus can result from de novo mutationsin the absence of affected parents. Once the mutation in present, the affectedindividual has a 50% possibility of transmitting the condition to futuregenerations.

A minority of birth defectsdless than 10%dare felt to be caused byenvironmental agents. Included in this group of defects are those relatedto conditions affecting the fetal environment, such as maternal diabetes;maternal use of alcohol, prescription drugs, and nonprescription drugs; con-genital infections; and mechanical constraint problems. These agents affectan embryo that would otherwise be destined to develop normally. Althoughenvironmental agents cause a minority of congenital anomalies, this is animportant group in that many of these extrinsic exposures are potentiallypreventable.

Prenatal detection of congenital anomalies

The increasing availability of prenatal genetic diagnosis has led to animportant role of the obstetrician/gynecologist in identifying couples atrisk to have a baby with an inherited disorder or congenital anomaly.This process of risk identification or screening is intended to identify preg-nancies where there is an increased possibility of congenital anomalies. Avariety of screening approaches are used, including patient medical and ex-posure history, family history, population-based carrier screening, maternalserum screening, and, finally, ultrasound, which can be both a screening tooland a diagnostic tool.

Family history

Obtaining a family history at the onset of prenatal care is both cost-effective and critical as an initial screen for genetic risk. Gatheringinformation for first-degree relatives (parents, siblings and offspring),second-degree relatives (uncles, aunts, grandparents, nieces, and nephews),and third-degree relatives (cousins) is recommended. Of particular concernis any history of a genetic diagnosis, recurrent pregnancy loss, birth defects,developmental delay, stillbirth, or other adverse reproductive outcomes.In addition, inquiring about a history of certain adult-onset disorders,such as thrombosis, familial cancers, infertility, psychiatric illness, autism,

438 RAPPAPORT

and behavioral disorders may be of significance. Information regardingethnic background, consanguinity, and potential teratogen exposures isalso collected as part of the genetic family screening interview. In clinicalpractice, drawing out a full pedigree and conducting a full genetic intervieware both tedious and time-consuming. An effective way to integrate geneticscreening into clinical practice is through the use of questionnaires or check-lists. A number of commercially available genetic screening tools aredesigned for the prenatal care setting. One example of a detailed historyform is available through the March of Dimes Web site (http://www.marchofdimes.com/professionals/15829_15858.asp). Another widely usedform in clinical practice can be obtained through the American College ofObstetricians and Gynecologists (ACOG).

Parental ages should be recorded as part of the initial family history.Advancing maternal age has a well-known association with increasing riskof chromosome aneuploidy, in particular autosomal trisomies (Table 1).Maternal age, however, is not associated with an increase in other

Table 1

Maternal age and risk of chromosome abnormalities at birth

Maternal age at delivery (y) Down syndrome All chromosome abnormalities

25 1 in 1250 1 in 476

26 1 in 1190 1 in 476

27 1 in 1111 1 in 455

28 1 in 1031 1 in 435

29 1 in 935 1 in 417

30 1 in 840 1 in 385

31 1 in 741 1 in 385

32 1 in 637 1 in 323

33 1 in 535 1 in 286

34 1 in 441 1 in 244

35 1 in 356 1 in 179

36 1 in 281 1 in 149

37 1 in 217 1 in 123

38 1 in 166 1 in 105

39 1 in 125 1 in 81

40 1 in 94 1 in 63

41 1 in 70 1 in 49

42 1 in 52 1 in 39

43 1 in 40 1 in 31

44 1 in 30 1 in 24

45 1 in 24 1 in 19

46 1 in 19 1 in 15

47 1 in 16 1 in 11

48 1 in 14 1 in 9

49 1 in 13 1 in 7

Data from Hook, EB. Chromosome abnormalities and spontaneous fetal death following

amniocentesis: further data and associations with maternal age. Am J Hum Genet 1983;

35:110–6.

439PRENATAL DIAGNOSIS AND GENETIC SCREENING

congenital anomalies in the absence of concurrent conditions, such as pre-gestational diabetes. Advanced paternal age has not been conclusivelyshown to be associated with an increase in aneuploidy risk. However, sev-eral population-based studies have suggested that advancing paternal ageover 45 is associated with an increased risk of a variety of structural and sin-gle-gene disorders. Reported birth defects include congenital heart defects,tracheoesophageal fistula, musculoskeletal anomalies, skeletal dysplasias,autism, schizophrenia, and such single-gene disorders as Marfan’s syn-drome, neurofibromatosis, osteogenesis imperfecta, and achondroplasia[2]. Although the risk for these disorders as a group may be as much asfour to five times higher than baseline, the absolute risk is small. It is esti-mated that paternal age effects add less than 1% risk to the backgroundrisk for a given pregnancy. Prenatal testing for the hundreds of potentiallyinvolved loci is not clinically available. However, targeted ultrasound maybe a useful screen to exclude structural birth defects.

Carrier screening for recessive conditions with increased prevalence incertain ethnic groups has become a standard part of prenatal care. Ethnicorigin should be recorded as part of the initial screen. Determining ethnicitycan be challenging because many patients do not have a clear idea of theirfamily origins. In addition, many individuals may identify multiple ethnic-ities in their background. A specific checklist that allows patients to list mul-tiple ethnicities is useful in this screening and is a part of most prenatalgenetic screening questionnaires.

A positive response on the screening questionnaire indicates a possiblerisk that needs further exploration (Box 1). Recurrent spontaneous abor-tions, stillbirths, and anomalous liveborns in the family history may indicatea familial chromosomal balanced translocation. Previous siblings with con-genital anomalies or a family history of anomalies may indicate a specificrisk that would require targeted testing. For example, a family history ofcongenital heart disease would suggest the need for a fetal echocardiogra-phy. A maternal family history of thrombosis, stroke, and recurrent preg-nancy loss may indicate an inherited thrombophilia that could affectpregnancy outcome. Genetic counseling and targeted ultrasound are alsogenerally recommended if there is a family history of structural birth defectsor a genetic disease. Referral for genetic consultation may also be warrantedfor potential teratogen exposures. Such exposures could be related to med-ications, infections, or maternal medical disease, such as pregestationaldiabetes.

Prenatal screening for specific genetic disorders

The ACOG currently recommends universal access to cystic fibrosis–car-rier screening as well as other carrier screening in specific high-risk groups.The purpose of carrier screening is to identify asymptomatic individuals

Box 1. Family and maternal history indicationsfor genetic counseling

Family history indications for genetics consultationPersonal or family history of a known or suspected genetic

disorder, birth defect, chromosome disorder, or metabolicdisorder

Family history of mental retardationFamilial chromosome rearrangementKnown carrier or family history of a genetic disorderUnexplained infertility, multiple pregnancy losses, or

unexplained previous stillbirthCongenital absence of the vas deferensPremature ovarian failure or elevated follicle-stimulating

hormone under the age of 40Family history of early-onset cancer or multiple family members

affected with early- or late-onset cancerPersonal or family history of stroke or blood clots before age 50

or known thrombophilia allele in other family members

Maternal medical indications for genetic consultationKnown maternal genetic disorderAdvanced maternal or paternal reproductive ageMaternal history of congenital anomaliesPregestational diabetesMaternal history of acute fatty liverMaternal seizure disorderExposure to drugs with known fetal risk (eg, isotretinoin, warfarin

lithium, tetracycline, seizure medications, statins,angiotensin-converting enzyme inhibitors)

Significant alcohol exposure (binge drinking or over two drinksa day)

Occupational exposure to lead, mercurySignificant radiation exposureCertain infectious diseases (eg, primary cytomegalovirus,

rubella, toxoplasmosis), primary varicella infection, parvovirus,sustained hyperthermia >2 days

440 RAPPAPORT

who are heterozygous carriers for common genetic disorders. For a condi-tion to be considered appropriate for population-based carrier screening,several criteria should be met. The natural history of the disorder shouldbe well understood and carry a potential for significant morbidity and mor-tality. The detection rate for disease-causing mutations should be greater

441PRENATAL DIAGNOSIS AND GENETIC SCREENING

than 90% or an allele frequency greater than 1% in the target population[3]. In addition, there must be adequate laboratory capacity to allow readyaccess to testing and there must be adequate resources for follow-up of pos-itive results.

Cystic fibrosis

Cystic fibrosis is the most common severe autosomal-recessive disease inindividuals of Northern European background, affecting about 30,000 indi-viduals in the United States [4]. Cystic fibrosis is caused by mutations in thecystic fibrosis transmembrane regulator gene leading to impaired chlorideconductance across cell membranes affecting primarily the pulmonary, di-gestive, and male reproductive systems. The disease is characterized bychronic pulmonary infections with progressive deterioration of lung func-tion. Early intervention and aggressive therapy to prevent deterioration oflung function is the cornerstone of treatment. Lung transplant has beenused increasingly for end-stage pulmonary disease. Pancreatic insufficiencyoccurs in about 85% of individuals with classic cystic fibrosis resulting inpoor weight gain and chronic malabsorption. In men, cystic fibrosis trans-membrane regulator gene mutations are associated with congenital bilateralabsence of the vas deferens and infertility. Women with cystic fibrosis areless likely to have infertility and are increasingly able to successfully com-plete pregnancy. With aggressive treatment to preserve lung function andpancreatic enzyme replacement, the life span for affected individuals isnow into the 30s and it is estimated that individuals born in the most recentdecade may live into their 40s [5]. About 15% of individuals with cystic fi-brosis have a milder disease course and a life span of about 56 years [6].

Over half of cystic fibrosis in the Northern European population iscaused by a single mutation known as the delta F508 allele. However,over 1000 disease-causing mutations have now been described [5]. Cystic fi-brosis occurs worldwide and has been described in individuals of every eth-nic background. Although cystic fibrosis is often described as a disease ofNorthern European populations, the incidence varies markedly from 1 in2500 for people of Northern European descent to 1 in 25,000 for individualsfrom Finland [7]. One of the highest incidences worldwide is found in NativeAmericans of Zuni Pueblo background, where the disease frequency is esti-mated at 1 in 333 [8]. The ability to detect mutations varies greatly betweenethnic groups and even within ethnic groups. Given this, the result of cysticfibrosis screening must always be described with the residual risk for the pa-tient’s particular ethnic background in mind (Table 2).

Prenatal screening for cystic fibrosis has been viewed as a model for theintegration of genetic testing into routine medical care [5]. In 2001, theACOG and the American College of Medical Genetics (ACMG) recommen-ded incorporating cystic fibrosis screening into clinical practice for both pre-natal and preconception screening for individuals of Northern European

Table 2

Carrier risks for cystic fibrosis before and after mutation testing with ACOG recommended

panel

Racial or ethnic group Detection rate

Estimated carrier risk

Before testing After negative test

Northern European or Caucasian 88% 1 in 25 1 in 200

Ashkenazi Jewish 94% 1 in 24 1 in 385

Hispanic American 72% 1 in 58 1 in 205

African American 64% 1 in 61 1 in 165

Asian American 49% 1 in 93 1 in 185

Data from American College of Medical Genetics. Technical standards and guidelines

for CFTR mutation testing. 2006 edition. Available at http://www.acmg.net/Pages/ACMG_

Activities/stds-2002/cf.htm. Accessed June 12, 2008.

442 RAPPAPORT

and Ashkenazi background. In 2005, the ACOG issued revised guidelinesbased on a review of the program to date. The current ACOG guidelinesrecommend offering cystic fibrosis screening to all patients regardless of eth-nic background. They also recommend making the patient aware of their re-sidual risk based on a best estimate of ethnic background. A critical aspectof cystic fibrosis screening is to provide appropriate counseling to patientsso that they are aware of the limitations of the testing and that a smallbut real residual risk for cystic fibrosis exists even after tests show negativefor carrying cystic fibrosis (see Table 2). Patient information pamphlets de-scribing cystic fibrosis screening are available from a variety of sources in-cluding the ACOG, the March of Dimes, and commercial laboratories.These written materials are useful in clinical practice to reinforce informa-tion and counseling points. Generally, prenatal testing is initiated with thepregnant woman followed by testing of the partner if a mutation is identi-fied. In cases where there are time constraints, concurrent testing of boththe partners may be indicated.

Initial ACOG/ACMG guidelines recommended a panel for identifying 25pan-ethnic mutations that were present in at least 0.1% of patients with cys-tic fibrosis. In the 2005 guidelines, two mutations were eliminated, leavinga recommended panel of 23 mutations. Many commercial laboratories,however, offer panels for detecting considerably more mutations. These ex-panded panels may be appropriate to enhance mutation detection in certainethnic groups. For example, using an expanded 86-mutation panel in theHispanic population may increase the detection rate to 78%, as opposedto 72% with the standard panel. The same expanded panel in the AfricanAmerican population increases the detection rate to 81%, as opposed to65% with the standard panel [9,10]. In obstetric practices with significantnumbers of patients of non-European background, screening with an en-hanced panel may beneficial. Complete sequence analysis of the cystic fibro-sis transmembrane regulator gene is possible although not recommended forcarrier screening. This technology is useful in evaluation of an individual

443PRENATAL DIAGNOSIS AND GENETIC SCREENING

with cystic fibrosis, a family history of cystic fibrosis, or a male with congen-ital bilateral absence of the vas deferens where a mutation cannot be de-tected on an expanded panel [5].

If a parent is found to carry a cystic fibrosis mutation, testing of the part-ner should be performed. If the partner’s test for carrying cystic fibrosis isnegative, the risk of an affected baby is very small but not zero. In addition,patients should be aware of the clinical consequences of the particular mu-tation they carry since not all cystic fibrosis mutations result in classical cys-tic fibrosis. Genetic consultation is recommended when a patient is found tocarry a cystic fibrosis mutation to address the issues of genotype/phenotypecorrelations and residual risk as well as to address other issues regarding no-tification of relatives, prenatal diagnostic testing, and the availability ofnewborn screening.

Tay-Sachs disease

Tay-Sachs disease is a lysosomal storage disease caused by a deficiency ofthe enzyme hexosaminidase A. The result of this enzyme deficiency is the ac-cumulation of GM2 gangliosides throughout the body, leading to a severeprogressive neurologic disorder. Infants affected with Tay-Sachs disease ap-pear normal at birth. However, by 5 to 6 months of age, their muscle tonedeteriorates, they fall short of developmental milestones, they develop men-tal retardation, and they become blind. They generally die by 6 years of age.There is no effective treatment for this disorder [11].

Tay-Sachs disease is an autosomal-recessive disorder and was one of theinitial disorders for which carrier screening was first clinically available. Thecarrier rate in Ashkenazi Jewish individuals is 1 in 30 as compared withthe background rate of 1 in 300. Successful application of carrier screeningprograms in the Ashkenazi Jewish population, initiated in the 1970s, haveresulted in a more than 90% decreased incidence of this disorder in theNorth American Jewish community. Today in North America, the vast ma-jority of children born with Tay-Sachs are of non-Jewish parents. Individ-uals of French Canadian and Cajun background have also been shown tohave higher carrier rates than those of the general population. TheACOG recommends that screening be offered to high-risk populations[11]. It has been suggested that this screening should be expanded to the gen-eral population because of the availability of a highly accurate, relatively in-expensive enzyme screen and the fact that the majority of children currentlyborn with Tay-Sachs disease are of non-Jewish parents [12].

Carrier testing can be performed by either DNA analysis or biochemicalanalysis. Clinical screening is often performed using both of these tech-niques. In the Jewish population, DNA analysis for the three most commonmutations detects 94% of carriers. Biochemical analysis for hexosaminidaseactivity will detect 98% of carriers. In the non-Jewish population, mutationtesting may detect fewer than 50% of carriers. Therefore, enzyme testing is

444 RAPPAPORT

recommended for screening of non-Jewish individuals. Of note, pregnancyand oral contraceptives result in decreased levels of hexosaminidase serumlevels. Serum screening in pregnancy may result in misclassification ofwomen as carriers. Therefore, if biochemical screening is performed inwomen who are pregnant or taking oral contraceptive, peripheral blood leu-kocyte testing must be used [11]. A further complication in Tay-Sachsdisease screening is that up to one third of the mutations identified innon-Jewish individuals are associated with a pseudodeficiency state in whichindividuals have a low activity level of hexosaminidase A when tested onconventional artificial substrate. However, these individuals are able to cat-alyze the breakdown of natural substrate GM2 ganglioside and are not atrisk of having a child with Tay-Sachs disease. Additional biochemical andDNA analysis is needed to clarify the results. Referral for genetic consulta-tion to review any ambiguous or positive screening results is highlyrecommended.

Other genetic disorders in individuals of Eastern EuropeanJewish descent

Subsequent to Tay-Sachs screening programs, a number of disordershave been identified in the Ashkenazi populations that meet criteria forscreening. These include disorders with a carrier rate of at least 1% in theAshkenazi population and a small number of common mutations leadingto carrier detection rates of over 90%. The disease incidence of these disor-ders ranges from 1 in 900 to 1 in 40,000 (Table 3). While individually theseare rare disorders, it is estimated that about one in five individuals of Ash-kenazi descent carries a mutation for at least one of these disorders. CurrentACOG/ACMG guidelines suggest offering screening for four of thesedisorders to couples where at least one partner is of Ashkenazi ancestry.

Table 3

Recessive disorders in individuals of Eastern European Jewish descent

Disorder Disease incidence Carrier frequency Detection rate

Tay-Sachs 1 in 3000 1 in 30 98% hexosaminidase A;

94% DNA based

Canavan disease 1 in 6400 1 in 40 98%

Cystic fibrosis 1 in 2500 1 in 29 97%

Familial dysautonomia 1 in 3600 1 in 32 99%

Fanconi anemia group C 1 in 32,000 1 in 89 99%

Neimann-Pick disease, A 1 in 32,000 1 in 90 95%

Mucolipidosis IV 1 in 65,000 1 in 127 95%

Bloom syndrome 1 in 40,000 1 in 100 95%–97%

Gaucher’s disease 1 in 900 1 in 15 95%

Data from March of Dimes genetic screening pocket facts. White Plains (NY): March of

Dimes; 2001.

445PRENATAL DIAGNOSIS AND GENETIC SCREENING

These four conditions are Tay-Sachs disease, cystic fibrosis, Canavan dis-ease, and familial dysautonomia [13].

Carrier screening tests are also available commercially for a number ofother disorders that are less prevalent in Ashkenazi populations but thatmeet screening criteria. These disorders include Fanconi anemia, Nei-mann-Pick disease, mucolipidosis IV and Bloom syndrome. All of these car-rier tests have a very high sensitivity in the Jewish population. However, thedetection rate in the non-Jewish population is unknown. When both parentsare of Ashkenazi descent and one has a positive carrier screen while theother parent is screen negative, the possibility of an affected child can be ex-cluded with high probability. In mixed couples, however, the ability to de-tect a carrier state in the non-Jewish partner is unknown, leading to anuncertain ability to exclude the possibility of having a child with the condi-tion. Couples of mixed background should be counseled regarding this un-certainty before embarking on screening for these disorders. Gaucher’sdisease is another disorder frequently included in commercially availableJewish genetic disorders panels. While the carrier frequency is high forthis disorderd1 in 15dthe clinical course of the disease is quite variable,ranging from a severe childhood disease to mild or unapparent disease inan adult. Effective treatment is available through enzyme-replacement ther-apy. Given the clinical spectrum, some couples may choose to declineGaucher’s disease screening even if they opt for the other carrier screens.Couples of Ashkenazi Jewish ancestry should be made aware of these ex-tended testing options either through educational material or genetic coun-seling. However, current ACOG standards do not recommend routinelyoffering this extended testing. Due to the complexity of this testing andthe many options to consider, referral for genetic counseling may be of ben-efit for couples considering extended testing [13].

Hereditary disorders of hemoglobin synthesis

Hereditary disorders of hemoglobin synthesis represent the most com-mon single-gene disorders in the world. It is estimated that more than 270million persons worldwide are heterozygous carriers of hereditary disordersof hemoglobin, and at least 300,000 affected homozygotes or compound het-erozygotes are born every year. The disorders include mutations causingstructurally abnormal hemoglobin, such as sickle cell disease, as well as dis-orders resulting in quantitative hemoglobin abnormalities, such as a- andb-thalassemia. The carrier frequency varies markedly worldwide. The high-est carrier rates are in individuals of Southeast Asian, African, or Mediter-ranean descent. Persons of northern European, Japanese, Native American,Inuit, or Korean descent are considered to be at low risk. Carrier screeningprograms for the hemoglobinopathies have been widely implemented inmany high-risk populations with variable success. The first nationwide pop-ulation-based carrier screening program in the United States was for sickle

446 RAPPAPORT

cell anemia and is widely viewed as a failure [14]. Eventually this programwas abandoned. The experience, however, provides insights into the poten-tial pitfalls of implementing population-wide carrier screening programs. Incontrast, carrier screening for b-thalassemia in Cyprus, while socially con-troversial, has resulted in the virtual elimination of affected homozygotenewborns in that region [15,16].

In the United States, identification of high-risk couples and voluntaryscreening are standard aspects of prenatal screening. High-risk groups in-clude those of African American, Southeast Asian, Chinese, or Mediterra-nean ancestry. The appropriate screening test for individuals identified ashigh risk for hemoglobin structural disorders is hemoglobin electrophoresis[17]. The hemoglobin S solubility testing to screen for sickle cell trait in per-sons of African background is not recommended because this will miss otherstructural hemoglobin variants as well as the thalassemia disorders, both ofwhich are in increased prevalence in this ethnic group. Mean corpuscularvolume (MCV) as an initial screen should be obtained for patients at in-creased risk for b- or a-thalassemia. Those who have an MCV level lessthan 80 mm3 maybe a carrier of one of the thalassemia traits, and should un-dergo hemoglobin electrophoresis. Elevated HbF and HbA2 greater than3.5% are associated with b-thalassemia. a-thalassemia carrier status canonly be detected through molecular genetic testing. When an MCV is belownormal, iron deficiency anemia has been excluded, and hemoglobin electro-phoresis is not consistent with b-thalassemia trait, molecular genetic testingshould be offered to detect a-globin gene deletions characteristic of a-thal-assemia [17]. Couples identified as carriers of structural or quantitative he-moglobin disorders should be referred to a prenatal genetics center forcounseling and possible diagnostic testing.

Fragile X syndrome

Fragile X was originally reported by Lubs [18] in 1969 as a syndrome ofX-linkedmale mental retardation. The condition is characterized by amarkeror fragile site on the X chromosome, which can be visualized when the cellsare grown in a low-folate cell culture environment. Subsequently the gene re-sponsible for this condition was identified as the fragile Xmental retardation-1 gene located on the X chromosome at Xq27.3. The gene is characterized bya repetitive CGG trinucleotide sequence at the 5’ promoter region. In thegeneral population, the CGG sequence is repeated from 6 to 50 times. In pre-mutation carriers, the CGG sequence repeats from 55 to 200 times. In fe-males, premutations are unstable and can undergo expansion duringoogenesis or postzygotic mitosis. This expansion results in CGG sequencesof 200 or more, which are considered full mutations and are clinically ex-pressed as fragile X syndrome. Expansion does not occur in male-to-maletransmission and to date there have been no children with a full mutation in-herited from a parent with a normal size allele [19].

447PRENATAL DIAGNOSIS AND GENETIC SCREENING

In an affected male, fragile X syndrome is classically associated with dis-tinctive facial features, such as large ears, a long face, a prominent forehead,prognathism, high arched palate, and, occasionally, cleft palate. The disor-der includes developmental delay and mild to severe mental retardation, at-tention deficit hyperactivity disorder, speech and language delay, anxiety,hand flapping, and autistic spectrum disorders. Fragile X syndrome is notconfined to males. Female full-mutation carriers have milder features thanmales but they also exhibit a similar range of physical characteristics andcognitive impairment. About 50% to 70% of females with full mutations ex-hibit IQs that are borderline or within the mentally retarded range. Femalefull-mutation carriers who have normal-range IQs may have learning dis-abilities or emotional problems, such as social anxiety, shyness, hyperactiv-ity, or impulsive behaviors [19].

Individuals with expanded repeat lengths varying from 50 to 200 repeatsdo not exhibit the classical fragile X syndrome phenotype, but are consid-ered fragile X premutation carriers. Recently, it has been recognized thatpremutation carriers may present with a spectrum of clinical findings.Mild manifestations of fragile X syndrome have been seen in male premuta-tion carriers. In addition, males over 50 may have a progressive neurodegen-erative disorder characterized by tremor, ataxia, Parkinsonism, andperipheral neuropathy known as fragile X–associated tremor/ataxia syn-drome. Women with premutations are usually unaffected intellectuallyand physically. However, they do have an increased risk for premature ovar-ian failure or ovarian dysfunction. Premutation alleles have been identifiedin about 2% of women with idiopathic premature ovarian failure and in14% of women with a family history of premature ovarian failure and noknown history of fragile X [20].

The prevalence of fragile X syndrome ranges from 1 in 4000 for malesand 1 in 6000 for females. In a recent large study looking at over 40,000asymptomatic women, the premutation carrier frequency was 1 in 154[21]. Given the prevalence of the premutations in the general population,the accuracy of detecting full mutations by DNA analysis, and the serious-ness of the clinical syndrome, some have suggested that general populationscreening for fragile X be offered in the United States. However, a numberof complexities in this screening make population-based implementationproblematic. Among the difficulties in offering population-based screeningare the complex multigenerational inheritance patterns, the variable pheno-type of full-mutation carriers, the fact that 50% of females with full muta-tions have normal IQs and may have only subtle cognitive features, thephenomena of contraction or reversion when an individual who carries anexpanded allele transmits a smaller allele to her offspring, and the significantnumber of women who would be identified with intermediate alleles. About1 in 52 women would likely carry high-intermediate alleles with 45 to 60CGG repeats. Although alleles containing fewer than 55 repeats are consid-ered stable, exceptions have been reported where expansion has occurred,

448 RAPPAPORT

making counseling very difficult for this potentially large group of women.Currently, fragile X screening is recommended in the preconception or pre-natal setting based on family history indications. This would includea known family history of fragile X as well as a family history of unex-plained mental retardation, developmental delay, autistic spectrum disor-ders, attention deficit disorders, and mental illness–personality disorders.A family history of premature ovarian failure, women under the age of 40with elevated follicle-stimulating hormone levels, and a family historywith late-onset tremor or ataxia of unknown origin should also raise clinicalsuspicion of fragile X [20]. Given the complexity of the phenotype of thisdisorder in relationship to DNA results, referral to a genetic counseling cen-ter experienced in prenatal genetics is recommended for this testing.

The role for preconception genetic screening

Carrier screening for single-gene disorders is often or perhaps always bestperformed before pregnancy. In addition, evaluation of couples with knownor possible genetic risk factors is also best undertaken before pregnancy.The identification of carrier status or other genetic risk can be very emotion-ally distressing, especially when this occurs during an ongoing pregnancy. Insome families, assessment of genetic risk may involve obtaining medical re-cords from affected individuals and genetic testing of multiple family mem-bers. The process of contacting family members, obtaining records, andanalyzing DNA mutations can be very time-consuming and complex, easilytaking 2 to 3 months to complete. When the evaluation is undertaken duringpregnancy, the results of this testing may not be available until the late thirdtrimester if at all. Resolution of these issues is much easier and less hurriedwithout the time limits, stress, and emotional turmoil of an advancingpregnancy.

Preconception carrier screening allows carrier couples to consider thefullest range of reproductive options. Knowledge of the risk of havingan affected child may influence a carrier couple’s decision to conceive.Some couples, after appropriate risk assessment and counseling, may de-cide that reproductive risks are too high to undertake pregnancy. Theywould have the option of avoiding pregnancy and adopting as an alterna-tive to undergoing pregnancy. Preimplantation genetic diagnoses are in-creasingly available. Using this technique with in vitro fertilization,embryos can be screened for the identified disorder so that only unaffectedembryos are implanted. The technique, while expensive because of the needfor in vitro fertilization, offers a possibility for couples at risk to havehealthy children while avoiding the need for abortion [22]. Other optionsinclude receiving gamete donations (sperm or egg donor), pursuing adop-tion, seeking a standard prenatal diagnosis, or accepting the genetic riskwithout further testing.

449PRENATAL DIAGNOSIS AND GENETIC SCREENING

Universal screening for birth defects in prenatal care

Universal screening for birth defects is now a standard aspect of prenatalcare. The two primary screening techniques are ultrasound and maternalserum screening. Recently, screening has focused on using an approachthat combines these two modalities.

Chromosome aneuploidy screening

Chromosome disorders affect approximately 1 in 500 newborns. Downsyndrome is the most common chromosome abnormality at birth and thesecond most common birth defect requiring hospital care at birth in theUnited States [23]. Prenatal chromosome analysis is highly accurate in de-tecting chromosome disorders before birth using such diagnostic testing pro-cedures as chorionic villus sampling in the first trimester and amniocentesisin the second and third trimester. These procedures, however, are expensive,invasive to the mother, and incur some degree of risk to the pregnancy.Given these factors, multiple strategies have been developed over the yearsto identify a population ‘‘at risk’’ that would have the most benefit fromdiagnostic testing.

The association of Down syndrome with maternal age has been recog-nized since the early 1900s [24]. In the 1970s, amniocentesis became techni-cally feasible in the second trimester and the risk was felt to be low enoughto offer this testing to women age 35 and over. This age cutoff was chosen torepresent 5% of the pregnant population, a 5% screen-positive rate (SPR).Maternal age, however, as a screen for Down syndrome has a low detectionrated30%dand is much less effective than any of the current serum or ul-trasound screening algorithms [25]. Despite this, the concept of maternal ageas the primary screen for Down syndrome has become firmly imbedded inboth the medical community and the lay community. Diagnostic testingwith chorionic villus sampling or amniocentesis for women over the ageof 35 represents the standard of care in many communities.

In the 1980s, open neural tube defect screening with maternal serum al-pha fetoprotein (MSAFP) was introduced to the United States and was of-ficially recommended by the ACOG as a component of prenatal care in1985. The association of low MSAFP with Down syndrome was recognizedearly in this screening program and led to the exploration of many differentserum and urinary markers for Down syndrome screening. From 1986 to1995, addition of the beta subunit of human chorionic gonadotropin(b-hCG), unconjugated estriol (uE3), and inhibin resulted in the doublemarker test (MSAFP and b-hCG), the triple marker test (MSAFP, b-hCG,and uE3), and quad marker test (MSAFP, b-hCG, uE3, and inhibin). Eachadditional biochemical marker increased the detection rates for Downsyndrome, although adding both cost and complexity to the biochemicalscreening.

450 RAPPAPORT

While the initial focus in biochemical screening was on the second trimes-ter, a variety of first-trimester markers have recently become available. Themost widely studied markers in the first trimester are free b-hCG, totalb-hCG, and pregnancy-associated plasma protein A (PAPP-A). PAPP-A,a glycoprotein produced by the trophoblast, is reduced in Down syndromeand trisomy 18 (Table 4). This marker is only useful as a quantitative screenin the first trimester. In contrast, b-hCG is the one marker that has beenfound to be useful in both the first and second trimester.

A major change in paradigm has been the use of ultrasound as a quantita-tive tool for first-trimester screening. Studies in the early and mid-1990s re-vealed a strong association between increasing nuchal translucency withincreasing risk of Down syndrome and other chromosome abnormalities[26–28]. The nuchal translucency is an area of fluid collection at the back ofthe neck of the embryo that can be observed in the first trimester and quanti-tatively measured from the 11th week to the completion of 13 weeks. Quanti-tativemeasurementsmust be performed according to standardized techniquesto achieve reproducible detection rates. Certification of competency and anongoing quality assurance program are critical for practitioners performingquantitative nuchal translucency (NT) screening. Seventy-five percent offetuses with Down syndrome were found to have a nuchal translucencygreater than the 95th percentile [26].When nuchal translucencymeasurementsare expressed asmultiple of themean they can be incorporated into a screeningprotocol much like other biochemical markers.

Screening algorithms

The availability of both first- and second-trimester serum markers forDown syndrome and the incorporation of ultrasound as a quantitativemarker have resulted in a proliferation of strategies and algorithms thatcould potentially be used for clinical screening. Several large multicenter tri-als, now completed, attempt to address this issue and to establish optimalapproach to screening (Table 5) [29–31].

Table 4

Relative changes in serum markers and nuchal translucency in trisomy 21, trisomy 18, and open

neural tube defects

Trisomy 21 Trisomy 18 Open neural tube defects

PAPP-A Y Y d

Free b-hCG [ Y d

Nuchal translucency [ [ dTotal b-hCG [ Y d

uE3 Y Y d

Alpha-fetoprotein Y Y [Inhibin [ d d

Table 5

Efficacy of various Down syndrome screening protocols at a 5% screen-positive rate

Screening test by trimester Detection rate for trisomy 21

First trimester

Maternal age 30%

Nuchal translucency measurement 64%–70%

Combined nuchal translucency, PAPP-A, free or total

b-hCG

82%–87%

Second trimester

Triple marker: MSAFP, uE3, total b-hCG 69%

Quad marker: MSAFP, uE3, total b-hCG, inhibin 81%

First and second trimester

Integrated screen 94%–96%

First trimester: nuchal translucency, PAPP-A

(not reported)

Second trimester: b-hCG, uE3, MSAFP, inhibin

Serum integrated screen 95%

First trimester: PAPP-A (not reported)

Second trimester: b-hCG, uE3, MSAFP, inhibin

Stepwise sequential 88%–94%

First trimester: nuchal translucency, PAPP-A, free

or total b-hCG (reported)

Second trimester: integrated screen

Contingent sequential 94%

First trimester: nuchal translucency, PAPP-A, free or

total b-hCG (reported)

Second trimester: screening only for intermediate-risk

group (no further testing for low-risk group)

Data from American College of Obstetricians and Gynecologists. ACOG practice bulletin.

Clinical management guidelines for obstetricians-gynecologists. Screening for fetal chromo-

somal abnormalities. Obstet Gynecol 2007;109(1):217–27.

451PRENATAL DIAGNOSIS AND GENETIC SCREENING

First-trimester only screening

Screening in the first trimester using nuchal translucency alone detects 75%of Down syndrome fetuses using a 5% SPR. If nuchal translucency is com-bined with serum markers PAPP-A and free b-hCG, the detection rate forDown syndrome is higherd82% to 87% [30]. The detection rate for other an-euploidies, such as T18, T13, and 45X, is about 78%. These detection rates aresimilar or slightly higher than those for quad screening. The advantages offirst-trimester screening are earlier diagnosis in affected cases and earlier reas-surance for patients who are anxious. Another advantage of first-trimesterscreening is accurate pregnancy dating. False-positive screens due to dating er-rors do not occur in first-trimester screening programs, whereas dating issuesare common in second-trimester screening programs and may double thefalse-positive rate in clinical practices where routine dating ultrasound isnot standard. Limitations of first-trimester screening are that nuchal tran-slucency measurements cannot always be obtained in all pregnancies, withmaternal habitus most often being the limiting factor. Access to nuchal trans-lucency–certified sonographers may also be a problem is some areas.

452 RAPPAPORT

Nuchal translucency screening is the only screening methodology thatevaluates each fetus individually. This is important in multiple gestations.In other screening methods, a single serum level is used to estimate risksfor two or more fetuses. In dizygotic twins with one affected fetus, the serummarker production masks the abnormalities in the affected fetus. The esti-mated efficacy of second-trimester screening in twin gestations using a 5%SPR is only 47% [32]. In contrast, a recent study of 535 twin gestations us-ing an SPR of 5% showed a detection rate of 83% for Down syndrome and67% for trisomy 18 using nuchal translucency alone. When using nuchaltranslucency plus serum markers, 100% of Down syndrome and trisomy18 cases were detected [33]. First-trimester screening with nuchal translu-cency and serum markers should be considered for twin gestations. Forhigher-level multiples, nuchal translucency alone is the only availablescreening modality available [34,35].

Abnormal first-trimester screening in chromosomally normal fetuses mayhave other implications for the pregnancy. Increased nuchal translucencyhas been associated with congenital heart defects [36], skeletal dysplasia,and other genetic disorders, as well increased risk of pregnancy loss, growthrestriction, and stillbirth [34,37]. Follow-up of pregnancies with increasednuchal translucency and normal chromosomes includes detailed fetal struc-tural ultrasound and echocardiogram, close follow-up for fetal growth, andcareful examination of the neonate at birth. Low PAPP-A has also been as-sociated with increased risk of stillbirth, particularly due to disorders asso-ciated with placental dysfunction, such as abruption and growth restriction[38]. Although clinical studies are lacking in terms of appropriate obstetricmanagement, following closely for growth, hypertensive disorders, and fetalwell-being may be reasonable.

Second-trimester serum screening

Second-trimester serum screening is performed between 15 and 21 weeksandmost commonly involves measurement ofMSAFP, uE3, and hCG (triplescreen); or those three markers plus inhibin (quad screen). Recent prospectivetrials have demonstrated increased detection rates with the quad screen [31].Therefore, for patients who initiate prenatal care after 14 weeks’ gestation,quad screening rather than triple screening is currently recommended [35].

Algorithms combining first- and second-trimester screening

While first- or second-trimester screening alone have advantages in someselect populations, several large studies have demonstrated that using an ap-proach that integrates first- and second-trimester screening markers resultsin optimal detection rates while minimizing the SPR [30,31]. Various optionsfor this type of combined first- and second-trimester screening have beenproposed and are now commercially available.

453PRENATAL DIAGNOSIS AND GENETIC SCREENING

Integrated screen

Integrated screening involves combining two parameters from the firsttrimesterdnuchal translucency and PAPP-Adwith four parameters fromthe second trimesterdalpha-fetoprotein, hCG, uE3, and inhibin. These sixparameters are analyzed using the maternal age as the a priori risk level.The screening result is not given until after the second-trimester blooddraw. This algorithm provides the highest detection rate with the lowestscreen-positive rate, 94% to 96% at 5% SPR. In areas where nuchal trans-lucency measurement is not available or cannot be obtained because of tech-nical factors, a serum-only integrated screen can be used. Serum-onlyintegrated screening has been trialed in clinical practice and shown tohave a Down syndrome detection rate of 79% to 87%. However, 13% offirst-trimester samples were not usable because of dating errors resultingin 13% of the population receiving quad marker results only [39]. The majorcriticism of integrated screening relates to the lack of a first-trimester result.The majority of Down syndrome and trisomy 18 conceptions are identifiedin the initial component of the screening. Withholding a risk estimate in thefirst trimester prevents women from accessing first-trimester diagnosis. Ac-cess to pregnancy termination is increasingly unavailable in the second tri-mester and the procedure has increasing medical risks later into thesecond trimester. In addition, studies indicate increased maternal anxietyand lack of bonding to the pregnancy until testing results are known. Ithas also been suggested that withholding first-trimester information withoutthe explicit consent of the patient may violate ethical principles involved inmedical informed consent [40].

Sequential screening

Sequential screening strategies are intended to address the issue of pro-viding first-trimester results while maintaining the high detection and lowSPR achieved by integrated screening. Stepwise sequential screening in-volves two steps. First-trimester nuchal translucency, PAPP-A, and freeb-hCG are obtained. A first-trimester risk level is calculated. If the risk isvery high after the first-trimester result, generally defined as 2% or greater,diagnostic testing is discussed and no further serum screening is performed.Less than 1% of the population would be in this group. For the remainderof the population, the first-trimester result is discussed. If they elect to con-tinue with screening, a second serum sample is collected at 15 to 20 weeksfor MSAFP, uE3, total b-hCG, and inhibin. Serum levels are then analyzedin conjunction with the first-trimester screen results (integrated screen anal-ysis), and a final risk estimate is obtained. Stepwise sequential screening dif-fers from integrated screening in that b-hCG is collected in both the first andsecond trimesters. This allows calculation of a first-trimester risk. Stepwisesequential also differs from simply ordering a first-trimester screen followed

454 RAPPAPORT

by a quad marker screen in that the first- and second-trimester markers areanalyzed together with a integrated computer algorithm. In quad markerscreening, the a priori risk level is automatically assigned as the maternalage–related risk. However, this is not the correct risk in a population wherefirst-trimester screening has already excluded most of the cases of Downsyndrome. The patient’s actual a priori risk after first-trimester screeningis not equivalent to her age-related risk. Ordering a quad marker test afterfirst-trimester screening results in an inaccurate risk assessment and in-creased screen-positive rates [41].

Contingency screening

While stepwise sequential screening has been shown to be highly effectivewith an acceptable SPR, the process of obtaining two sets of laboratorytests and two sets of calculations for each patient is expensive and time-consuming. In addition, retesting patients with a very low risk level afterthe first-trimester screen may not significantly increase the detection rate forDown syndrome. Contingent sequential screening is an algorithm in whichthe second-trimester follow-up is contingent upon the first-trimester results.Contingency screening uses the same process as stepwise sequential. Afterfirst-trimester results are obtained, the results are discussed with the patient.In the very high risk group, diagnostic testing is discussed andno further serumtesting is indicated. In contingent sequential screening a very low risk group isalso defined. In this group, the results of the first-trimester risk assessment arediscussed and the patient is given the option of diagnostic testing.However, nofurther serum testing is performed.Depending on the low-risk cutoff used, thisgroup would encompass 70% to 75% of the population. For patients in themiddle range, representing 25% to 30% of the population, serum is obtainedin the second trimester for an integrated screening result. This approach hasbeen suggested to be optimal in terms of screening costs and outcomes [42].However, clinical studies are still pending to look at the feasibility of applyingthis somewhat complicated algorithm in routine clinical practice.

American College of Obstetricians and Gynecologists guidelines

for chromosome screening

Implementing chromosome screening into routine practice is challenginggiven the array of options and lack of consensus on the ideal screening pro-tocol. In January 2007, the ACOG released updated clinical guidelines toaddress these uncertainties and suggest appropriate steps for implementingnew algorithms in clinical practice. Some of these suggested guidelines forclinical practice are as follows:

� All women who initiate care before 20 weeks’ gestation should be offeredscreening, regardless of maternal age.

455PRENATAL DIAGNOSIS AND GENETIC SCREENING

� It is not necessary to offer all screening options to all patients. Eachpractice should identify the strategy that best meets the needs of itspatients and uses resources available in the community.� Women who are seen in the first trimester should be offered a strategythat combines both first- and second-trimester screening. Women firstseen in the second trimester should be offered quad screening.� Regardless of the testing selected, patients should receive informationabout detection rates, false-positive rates, advantages, and limitationsso they can make an informed decision.� Risk levels should be communicated as a numeric risk rather than a pos-itive/negative result.� Both screening and diagnostic testing should be available to all womenwho present for care before 20 weeks’ gestation, regardless of risk levelsor maternal age [35].

Screening for neural tube defects

Universal screening for neural tube defects was the original purpose ofmaternal serum screening programs and remains an important aspect of pre-natal screening. MSAFP has been used as a universal screen for neural tubedefects since the 1980s. The detection rate of MSAFP screening programs is75% to 80%. The majority of encephaloceles and 30% to 40% of myelome-ningoceles are missed. High-resolution ultrasound has been proposed as analternative and better screen for neural tube defects. The sensitivity of ultra-sound for the detection of neural tube defects has been reported to be ashigh 94% to 100% [41]. While the detection rate is expected to be higherin a perinatal referral center, one report looking at outcomes with routineultrasound in a scanning center found a 96% detection rate [43]. Offeringscreening for neural tube defects is recommended for all prenatal patients[35]. High-resolution ultrasound is an alternative to MSAFP in practiceswhere this is available.

Diagnostic testing

The goal of screening is to identify women who would benefit most fromdiagnostic testing and to decrease the need for invasive procedures, resultingin decreased cost and procedure-related losses. However, the concept of us-ing set thresholds for offering diagnostic testing has been questioned.Screening does not detect all fetuses with aneuploidy and will not detectmost fetuses with other types of chromosome disorders. The risk level forinvasive testing is much lower in recent studies than has been traditionallyreported [44,45]. In addition, women perceive their own risk as well as therisk of having an affected baby differently and individually. This has ledto questions regarding the ethics of using threshold-based cutoffs in thatthis may undermine the individual autonomy of the pregnant woman to

456 RAPPAPORT

make her own best decision based on her personal perceived risk [46]. In rec-ognition of these concerns, the ACOG currently recommends that all pa-tients be informed of the option of diagnostic testing as an alternative toscreening, regardless of maternal age of risk levels [35].

Future directions

The rapid transition of genetic research from laboratory to clinical appli-cations, driven in part by an aggressive biomedical industry, assures that thenumber of options available for directed and universal screening in precon-ception and pregnancy care will continue to expand. Discussions are cur-rently underway for consideration of universal screening for fragile X,spinal muscular atrophy, and Tay-Sachs disease. The use of ultrasound asa screening and diagnostic tool will continue to expand. Other first-trimesterand second-trimester ultrasonographic markers have been proposed and arebeing assessed for clinical utility. New laboratory techniques may enhancethe diagnostic capabilities of chorionic villus sampling and amniocentesis.Comparative genomic hybridization (CGH) is being used increasingly asan adjuvant to standard karyotype to diagnosis microdeletion/duplicationdisorders or as a general screen in fetuses with anomalies where standardchromosome testing is normal. CGH testing, which is already being usedpostnatally, detects significantly more and smaller changes in the amountof chromosomal material in significantly less time than a standard chromo-some karyotype would take and may eventually replace standard techniquesfor cytogenetic analysis. Finally, advances in in utero therapy and special-ized delivery planning time are starting to provide options for selected dis-orders [47,48]. These advances may bring about a transition in the futurefrom an abortion-based mindset to realistic treatment-based options for fe-tuses affected with congenital disorders.

References

[1] SilverRM,VarnerMW,ReddyU, et al.Work-up of stillbirth: a review of the evidence. Am J

Obstet Gynecol 2007;196(5):433–44.

[2] Lazarou S,Morgentaler A. The effect of aging on spermatogenesis and pregnancy outcomes.

Urol Clin North Am 2008;35(2):331–9.

[3] Gross SJ, Pletcher BA,MonaghanKG. Carrier screening in individuals of Ashkenazi Jewish

descent. Genet Med 2008;10(1):54–6.

[4] Foundation CF. Annual Patient Registry Database. 2002.

[5] Langfelder-Schwind E, Kloza E, Sugarman E. Cystic fibrosis prenatal screening in genetic

counseling practice: recommendations of the National Society of Genetic Counselors.

J Genet Couns 2005;14(1):1–15.

[6] ACOG Committee Opinion. Number 325, December 2005. Update on carrier screening for

cystic fibrosis. Obstet Gynecol 2005;106(6):1465–8.

[7] Kere J, Estivill X, Chillon M, et al. Cystic fibrosis in a low-incidence population: two major

mutations in Finland. Hum Genet 1994;93(2):162–6.

457PRENATAL DIAGNOSIS AND GENETIC SCREENING

[8] Kessler D, Moehlenkamp C, Kaplan G. Determination of cystic fibrosis carrier frequency

for Zuni native Americans of New Mexico. Clin Genet 1996;49(2):95–7.

[9] Heim RA, Sugarman EA, Allitto BA. Improved detection of cystic fibrosis mutations in the

heterogeneous U.S. population using an expanded, pan-ethnic mutation panel. Genet Med

2001;3(3):168–76.

[10] Sugarman EA, Rohlfs EM, Silverman LM, et al. CFTR mutation distribution among U.S.

Hispanic and African American individuals: evaluation in cystic fibrosis patient and carrier

screening populations. Genet Med 2004;l6(5):392–9.

[11] ACOG committee opinion. Number 318, October 2005. Screening for Tay-Sachs disease.

Obstet Gynecol 2005;106(4):893–4.

[12] Roe AM, Shur N. From new screens to discovered genes: the successful past and promising

present of single gene disorders. Am J Med Genet C Semin Med Genet 2007;145(1):77–86.

[13] ACOG committee opinion. Number 298, August 2004. Prenatal and preconceptional carrier

screening for genetic diseases in individuals of Eastern European Jewish descent. Obstet

Gynecol 2004;104(2):425–8.

[14] Lee T. Gene therapy: the promise and perils of the new biology. New York: Plenum Press;

1993.

[15] Angastiniotis M. Management of thalassemia in Cyprus. Birth Defects Orig Artic Ser 1992;

28(3):38–43.

[16] Angastiniotis M, Modell B, Englezos P, et al. Prevention and control of haemoglobinopa-

thies. Bull World Health Organ 1995;73(3):375–86.

[17] ACOG (American College of Obstetricians and Gynecologists) committee opinion. Genetic

screening for hemoglobinopathies, number 238, July 2000 (replaces number 168, February

1996). Committee on Genetics. Int J Gynaecol Obstet 2001;74(3):309–10.

[18] Lubs HA. A marker X chromosome. Am J Hum Genet 1969;21(3):231–44.

[19] McConkie-Rosell A, Finucane B, Cronister A, et al. Genetic counseling for fragile X syn-

drome: updated recommendations of the National Society of Genetic Counselors. J Genet

Couns 2005;14(4):249–70.

[20] McConkie-Rosell A, Abrams L, Finucane B. Recommendations from multi-disciplinary

focus groups on cascade testing and genetic counseling for fragile X–associated disorders.

J Genet Couns 2007;16(5):593–606.

[21] BerkenstadtM, Ries-Levavi L, Cuckle H. Preconceptional and prenatal screening for fragile

X syndrome: experience with 40,000 tests. Prenat Diagn 2007;27(11):991–4.

[22] Bick DP, Lau EC. Preimplantation genetic diagnosis. Pediatr Clin North Am 2006;53(4):

559–77.

[23] Hospital stays, hospital charges, and in-hospital deaths among infants with selected birth

defects–United States, 2003. MMWR Morb Mortal Wkly Rep 2007;56(2):25–9.

[24] Nicolaides KH. Screening for fetal chromosomal abnormalities: need to change the rules.

Ultrasound Obstet Gynecol 1994;4(5):353–4.

[25] Rose NC. On the current dilemma of Down syndrome screening. Obstet Gynecol 2006;

107(1):2–3.

[26] Nicolaides KH, BrizotML, Snijders RJ. Fetal nuchal translucency: ultrasound screening for

fetal trisomy in the first trimester of pregnancy. Br J Obstet Gynaecol 1994;101(9):782–6.

[27] Pandya PP, Brizot ML, Kuhn P. First-trimester fetal nuchal translucency thickness and risk

for trisomies. Obstet Gynecol 1994;84(3):420–3.

[28] Pandya PP, Snijders RJ, Johnson SP. Screening for fetal trisomies by maternal age and fetal

nuchal translucency thickness at 10 to 14 weeks of gestation. Br J Obstet Gynaecol 1995;

102(12):957–62.

[29] Wald NJ, Rodeck C, Hackshaw AK, et al. SURUSS in perspective. Semin Perinatol 2005;

29(4):225–35.

[30] Wapner RJ. First trimester screening: the BUN study. Semin Perinatol 2005;29(4):236–9.

[31] Malone FD, Canick JA, Ball RH. First-trimester or second-trimester screening, or both, for

Down’s syndrome. N Engl J Med 2005;353(19):2001–11.

458 RAPPAPORT

[32] Cuckle H. Down’s syndrome screening in twins. J Med Screen 1998;5(1):3–4.

[33] Chasen ST, Perni SC, Kalish RB. First-trimester risk assessment for trisomies 21 and 18 in

twin pregnancy. Am J Obstet Gynecol 2007;197(4):374, e1–3.

[34] BilardoCM, Pajkrt E, deGraaf I. Outcome of fetuses with enlarged nuchal translucency and

normal karyotype. Ultrasound Obstet Gynecol 1998;11(6):401–6.

[35] ACOG Practice Bulletin No. 77: screening for fetal chromosomal abnormalities. Obstet

Gynecol 2007;109(1):217–27.

[36] Makrydimas G, Sotiriadis A, Huggon IC. Nuchal translucency and fetal cardiac defects:

a pooled analysis of major fetal echocardiography centers. Am J Obstet Gynecol 2005;

192(1):89–95.

[37] Souka AP, Snijders RJ, Novakov A. Defects and syndromes in chromosomally normal

fetuseswith increased nuchal translucency thickness at 10–14weeks of gestation.Ultrasound

Obstet Gynecol 1998;11(6):391–400.

[38] Smith GC, Crossley JA, Aitken DA. First-trimester placentation and the risk of antepartum

stillbirth. JAMA 2004;292(18):2249–54.

[39] Knight GJ, Palomaki GE, Neveux LM. Integrated serum screening for Down syndrome in

primary obstetric practice. Prenat Diagn 2005;25(12):1162–7.

[40] Chervenak FA,McCullough LB, Chasen ST. Clinical implications of the ethics of informed

consent for first-trimester risk assessment for trisomy 21. Semin Perinatol 2005;29(4):277–9.

[41] Norton ME. Genetic screening and counseling. Curr Opin Obstet Gynecol 2008;20(2):

157–63.

[42] Ball RH, CaugheyAB,Malone FD. First- and second-trimester evaluation of risk for Down

syndrome. Obstet Gynecol 2007;110(1):10–7.

[43] Norem CT, Schoen EJ, Walton DL. Routine ultrasonography compared with maternal

serum alpha-fetoprotein for neural tube defect screening. Obstet Gynecol 2005;106(4):

747–52.

[44] Eddleman KA, Malone FD, Sullivan L. Pregnancy loss rates after midtrimester amniocen-

tesis. Obstet Gynecol 2006;108(5):1067–72.

[45] Kong CW, Leung TN, Leung TY. Risk factors for procedure-related fetal losses after mid-

trimester genetic amniocentesis. Prenat Diagn 2006;26(10):925–30.

[46] SharmaG,McCullough LB, Chervenak FA. Ethical considerations of early (first vs. second

trimester) risk assessment disclosure for trisomy 21 and patient choice in screening versus

diagnostic testing. Am J Med Genet C Semin Med Genet 2007;145(1):99–104.

[47] Coleman BG, Adzick NS, Crombleholme TM. Fetal therapy: state of the art. J Ultrasound

Med 2002;21(11):1257–88.

[48] Kuller JA, Katz VL, Wells SR. Cesarean delivery for fetal malformations. Obstet Gynecol

Surv 1996;51(6):371–5.