screening for down syndrome

Dr nadia munir Tmo gynae c unit

LRH

Prenatal screening For down syndrome

contentsDefinationSignificance BackgroundIncidenceAntenatal screening First trimester screeningSecond trimester screening Down syndromeConclusion Take home message

The detection of abnormalities in the fetus, before birth’

Screening is the process of surveying a population, using a specific marker or markers to identify the individuals in the population at higher risk for a particular disorder.

Definition:

The purpose of prenatal diagnosis is not simply to detect abnormalities in fetal life and allow termination.It rather have following goals :

Provides information. Provide reassurance & remove anxiety. Confirmatory testing. Plan for appropriate management . Prenatal treatment of the fetus. Routinely offered. Always explain pros and cons.

High prevalence in the population. Knowledge about the disease and its progression. Highly sensitive and specific. Acceptable to population. Availability of other diagnostic test. Problem solution. Cost effective.

Principles for screening

National screening programmes were first introduced in UK 1996 and these includes :-

Screening for down syndrome. Fetal anomaly screening. Screening for sickle cell anemia and

thalassaemia. Screening for infectious diseases.

Background

DOWN SYNDROME John Langdon Down in 1866. 1930s research declared ____

chromosomal defect 1959,Jerome Lejune, discovered

an extra chromosome (trisomy) in DS individuals

Down syndrome is a chromosomal disorder. Error in cell division, the likelihood of such an

error occuring increases with maternal age.

This means that an older mother is more likely to give birth to a child with Down syndrome than her younger counterpart.

Incidence of Down’s Syndrome

The likelihood of a woman under 30 years of age giving birth to a child with Down syndrome is less than 1:1000, but increases the older the woman gets , with an incidence of about 1:100 and 1:30 at 40 and 45 years of age respectively.

STATISTICS

6000 infants/year are born with down syndrome

1979-2010 incidence is increased by 30%

In 2010,_____USA was about 250,700.

Genetic testing is on increase 5.3% of pregnancies with DS are conti… 90% of pregnancies e DS terminated in New

Zealand 92% in USA 93% in UK

STATISTICS….

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INDICATIONS OF PRENATAL SCREENING

1. Advanced maternal age.2. Previous child with a chromosomal abnormality.3. Family history of a chromosomal abnormality.4. Family history of a single gene disorder.5. Family history of other congenital structural

abnormality.6. Abnormalities identified in pregnancy.7. Other risk factors(consanguinity,poor obs.

History,maternal history)

Time • 1st trimester• 2nd trimester

Technique• Non .Invasive• invasive

CLASSIFICATION….

Screening tests for down syndrome

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METHODS OF PRENATAL SCREENING

NON INVASIVE TECHNIQUES

Fetal visualization Maternal serum

screening Separation of fetal

cells from the mother's blood

INVASIVE TECHNIQUES

Fetal visualization Fetal tissue

sampling Cytogenetics Molecular genetics

First Trimester ScreeningScreening methods-1. Age of mother.2. Ultrasonography 3. Maternal serum markers.4. Ductus venosus sonography.5. Tricuspid Regurgitation Doppler study.

Secondaryscreening tools

The only screening marker in 1970s

Age >35 years, amniocentesis was offered

<1/3rd of fetuses with trisomy 21 were identified

Maternal age

MATERNAL AGE RISK OF DS(newborn)

20years 1/1667

25years 1/1250

30 years 1/1000

35 years 1/385

40 years 1/100

45 years 1/30

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FETAL VISUALISATION

ULTRASONOGRAPHY :

The developing embryo can first be visualized at about 6 weeks gestation.

Recognition of the major internal organs & extremities to determine if any are abnormal can best be accomplished between 16 to 20 weeks gestation.

Nuchal Translucency

• Nuchal Translucency(NT) refers to the normal subcutaneous fluid filled space between the back of the neck and underlying skin.

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2D US

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2D US

Nuchal TranslucencyNT measurement Chance of normal birth

≤ 3.4mm 95%

3.5 – 4.4mm 70-86%

4.5 – 5.4mm 50-77%

5.5 – 6.4mm 67%

≥ 6.5mm 31%

NT also increased in:

Trisomies 21, 18, 13.

Spontaneous fetal loss

Cardiac defects, diaphragmatic hernia, pulmonary defects.

skeletal dysplasias, congenital infection.

Normal pregnancy .

Nasal Bone (NB) Absent nasal bone on USG done at 11 to 13 weeks is another marker of Downs syndrome.

Absence of Nasal bone is not related to NT and can be combined in one scan. Detection rate of DS is 67%. When combined with NT detection rate is 90%.

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2D US

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2D US

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3D & 4D US In recent years three-dimensional ultrasound (3D) &

four-dimensional ultrasound (4D) have started to play an increasing role in prenatal diagnosis. They can be applied in assessing facial features, central nervous system abnormalities and skeletal defects

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Limitations in this procedure

Findings are based upon views of the fetus, the estimated gestational age, sonographer experience, & the degree of anomaly severity.

• Congenital heart lesions as early as 17-19 wk of gestation. • When this technique is used with duplex or color flow

Doppler, it can identify a number of major structural cardiac defects &• rhythm.• 59% to 93% of aneuploid fetus has abnormal ductus

venosus flow.• Down syndrome detection rate during 11 to 13 weeks is 68%

alone.

FETAL ECHO

First trimester Ductus venosus sonography.

• 1st Trimester Ductus venosus blood flow is an adjunctive test For fetal aneuploidy screening.• Forward triphasic pulsatile flow is normal. •Reversed flow at time of atrial contraction is abnormal.

First trimester Tricuspid Regurgitation(TR)

• Abnormal tricuspid regurgitation in 1st trimester is associated with fetal aneupoidy.• TR ______ if a regurgitant jet of at least 60cm/sec is noted extending to over half of systole.• Down syndrome detection rate during 11 to 13 weeks is 68% alone. • Not recommended for routine screening.• Has a role as second line screening test with detection rate of 92%.

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MATERNAL SERUM SCREENING

Maternal serum screening is used to identify women at increased risk of having a child with trisomies 18 or 21

or and open neural tube defect (NTD), while posing no risk to the pregnancy.

Screening in the first trimester involves the measurement of PAPP & free b HCG levels in

maternal serum.

Comparison of Prenatal Screening Tests in Detecting Down Syndrome

Tests Detection Rate(DR)

False Positive Rate (FPR)

IPS 85 - 90% 2 - 4%

FTS 78 - 85% 3 - 9%

Quad 75 - 85% 5 - 10%

Triple 60 - 85% 5 - 12%

NT alone 60 - 70% 5%

Triple testDisorders MSAFP uE3 Beta hCG Inhibin A

Open NTD increased No change No change No change

Downs syndrome

decreased decreased increased Increased

Trisomy 18 decreased decreased No change No change

a protein synthesized by the fetus is detectable in maternal serum from week 6 of pregnancy,with a peak in week 34 of gestation (4 mg / ml), its value decreasing in 8-12 months after birth. Measurement of alpha-fetoprotein can be done from amniotic fluid or from maternal blood.

BIOCHEMICAL MARKERS PRENATAL DIAGNOSIS 1- Alpha-fetoprotein (AFP)

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Contin... AFP is produced by the yolk sac & later by the liver; it

enters the amniotic fluid & then the maternal serum via fetal urine.

In the condition of an open NTD (eg, anencephaly, spina bifida) & abdominal wall defects in the fetus, AFP diffuses rapidly from exposed fetal tissues into amniotic fluid, and the MSAFP level rises

Conditions associated with abnormal level of MSAFPELEVATED LEVELS• Underestimation of gestation age. • Multifetal gestation.• IUD• NTD• Gastroschisis and omphalocele• Esophageal or intestinal obstruction.• sacrococcygeal teratoma.• Renal anomalies

LOW LEVELS• Overestimated gestational age•Obesty•Down syndrome•GTD•Fetal death

The hormone human chorionic gonadotropin is produced during pregnancy by synctiotrophoblast.

Its levels will double every 72 hours. The level will reach its peak in the first 8 – 11 weeks of pregnancy and then will decline and level off for the remainder of the pregnancy.

Human Chorionic Gonadotropin

3 weeks LMP: 5 – 50 mIU/ml 4 weeks LMP: 5 – 426 mIU/ml 5 weeks LMP: 18 – 7,340 mIU/ml 6 weeks LMP: 1,080 – 56,500 mIU/ml 7 – 8 weeks LMP: 7, 650 – 229,000 mIU/ml 9 – 12 weeks LMP: 25,700 – 288,000 mIU/ml 13 – 16 weeks LMP: 13,300 – 254,000 mIU/ml 17 – 24 weeks LMP: 4,060 – 165,400 mIU/ml 25 – 40 weeks LMP: 3,640 – 117,000 mIU/ml Non-pregnant females: <5.0 mIU/ml

Guideline to HCG levels during pregnancy/ LMP Last menstrual period

Estriol (E3) is one of the three major naturally occurring estrogens.

During pregnancy, estriol is secreted by the placenta and fetus and becomes the dominant estrogen form.

The primary form of estriol measured during pregnancy is unconjugated estriol .

Used in the evaluation of pregnancy complications.

Unconjugated estriol

Glycoprotein hormones .

Its a better marker of placental function than hcG because of its shorter half-life.

The measurement of inhibin A in early pregnancy could be in predicting miscarriage, Down's syndrome, preeclampsia, and fetal growth restriction in the first and/or second trimester.

Inhibin - A

It largest proteins produced by both the embryo and the placenta.

This protein have several different functions, including preventing recognition of the fetus by the maternal immune system, matrix mineralization and angiogenesis.

Detection of this protein helps in first and second trimester diagnostic test for aneuploidies, including Trisomies 21 or Down’s Syndrome

Pregnancy-associated plasma protein-A

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Separation of fetal cells from the mother's blood

It involves isolating fetal cells from maternal blood to analyse fetal chromosomes and/or DNA. Ordinarily, only a very small number of fetal cells enter the maternal circulation; but once they enter,can be readily identified.

These cells can be collected safely from approximately 12-18 weeks' gestation onward.

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Fetal blood cells can then be analyzed for the diagnosis of genetic disorders using FISH, PCR etc.

Fetal cells separated from a mother's blood have been successfully used in the diagnosis of cystic fibrosis, sickle cell anemia, and thalassemia in a fetus.

A. Maternal RBCs B. Fetal RBCs (nucleated)

Invasive • Amniocentesis• Chorionic villous sampling

Invasiveness..

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INVASIVE TECHNIQUES Pre procedure genetic counseling- Pre procedure counseling is necessary as it will allow

patients to understand their situation allow them to give consent(or withhold) for the procedure.

Following points to be explained—

1. The chance that fetus will be affected.2. Nature and consequences of disorder.3. Risks and limitation of procedure.4. Time required for reporting.5. Possibility of complications.

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Fetal Tissue Sampling Amniocentesis Chorionic villus sampling (CVS) Percutaneous umbilical blood sampling (PUBS) Percutaneous skin biopsy Other organ biopsies, including muscle & liver

biopsy

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Amniocentesis : performed at 10-14 weeks but usually done at 15-18

weeks of gestation. pregnancy loss rate of 1 – 2 % & an increased

incidence of clubfoot.

A 22-gauge needle is passed through the mother's lower abdomen into the amniotic cavity inside the uterus, & 10-20 mL of amniotic fluid that contains cells from amnion, fetal skin, fetal lungs, and urinary tract epithelium are collected.

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1. The Cells are grown in culture for chromosomal, biochemical,

& molecular biologic analyses.2. The Supernatant amniotic fluid is used for the

measurement of substances such as AFP, AChE,bilirubin & pulmonary surfactant

3. In the third trimester of pregnancy, the amniotic fluid can be analyzed for determination of fetal lung maturity.

The results of cytogenetic and biochemical studies on amniotic cell cultures are more than 90% accurate.

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Chorionic villous sampling Under USG guidance, a sample of placental tissue is

obtained through a catheter places either transcervically or transabdominally.

Performed at or after 10 wks’ gestation, CVS provides the earliest possible detection of a

genetically abnormal fetus through analysis of trophoblast cells.

Transabdominal CVS can also be used as late as the 3rd trimester when amniotic fluid is not available or when fetal blood sampling cannot be performed.

CVS, if preformed before 10 wks’ gestation , can be ass. with an increased risk of fetal limb reduction defects & oromandibular malformations.

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Direct preparations of rapidly dividing cytotrophoblasts

can be prepared, making a full karyotype analysis available in 2 days.

Although direct preparation minimize maternal cell contamination, most centers also analyse cultured trophoblast cells, which are embryologically closer to the fetus.This procedure takes 8 – 12 days.

In approximately 2% of CVS samples, both karyotypically normal & abnormal cells are identified. Because CVS acquired cells reflect placental constitution, in these cases, amniocentesis is typically performed as a followup study to analyze fetal cells. Approximatally 1/3rd of CVS mosaicisms are confirmed in the fetus through amniocentesis.

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Cytogenetic Investigations

Chromosome Analysis ( Karyotype Analysis )

Fluorescence in situ Hybridization (FISH)

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Direct DNA AnalysisLinkage Analysis(indirect DNA analysis)

DNA Sequencing

Molecular genetics

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PRENATAL TREATMENT In the most situations the diagnosis of prenatal

abnormalities has a subsequent option of termination of the pregnancy.

While this applies in most situations, there is cautious optimism that with the advent of gene therapy prenatal diagnosis will, in time, lead to effective treatment in utero.

Genetic counseling • Clinical Genetics is the discipline concerned with the diagnosis and management of the medical, social, and psychological aspect of hereditary diseases.• Genetic counseling is an integral part of genetic testing /prenatal Testing.• The process of genetic counseling involves components like

1)validation of diagnosis2)obtaining the family history, 3)estimation of risk of recurrence 4)and by all these helping the family to reach

decisions and take appropriate action and follow up.• Nondirective counseling is adopted as standard, In this patients are not told what decision to make with regards to testing and management options but, instead, are provided information and support.