Prenatal Development & Birth

Chapter Three

Conception & Genetics

• When does life begin?

• Process of Conception:– Every 28 days egg is

released, which contains 23 chromosomes

– Gametes – cells that have only 23 chromosomes.

• Rx indicates that only about half of all conceptions are likely to survive to birth. What are some reasons for this large number of fertilized eggs not surviving?

• Male gamete + female gamete = zygote

Conception

Two Types of Chromosomes:• 22 of the pairs of

chromosomes are called autosomes

• Sex chromosomes = X or Y– Female has two X

chromosomes (XX)– Male has one X and one Y– Gender of a child is

determined by the sex chromosome from the sperm (because mom only has X chromosomes)

• Chromosomes are comprised of DNA (deoxyribonucleic acid) that is further subdivided into genes.

Twins: Identical

Monozygotic Twins

• Aka= identical twins• A single egg is fertilized to form one

zygote which then divides into two separate embryos.

• Odds of having identical twins are about 4 in 1,000

• Same sex (gender)

Twins: Fraternal

Dizygotic Twins

• Aka = fraternal twins• Two eggs are fertilized by two separate sperm.• Same sex or opposite sex

– Number of multiple births has increased 65% over last 25 years

• Women 35+ more likely to conceive twins & other multiples

• Women 35+ are more likely to be treated with fertility drugs

Genetics

• Genotype – specific genetic material on an indiv’s chromosomes.

• Phenotype – the observed characteristic of the indiv (brown eyes; blonde hair)

DOMINANT AND RECESSIVE CHARACTERISTICSCharacteristics in the left-hand column dominate over those characteristics listed in the right-hand column.  

 DOMINANT TRAITS RECESSIVE TRAITS

eye coloring brown eyesgrey, green, hazel, blue

eyes

vision farsightednessnormal visionnormal visionnormal vision

normal visionnearsightednessnight blindnesscolor blindness*

hair dark hairnon-red haircurly hairfull head of hairwidow's peak

blonde, light, red hairred hairstraight hairbaldness*normal hairline

facial features

Other

dimplesunattached earlobesfrecklesbroad lips

Immunity to poison ivyNormal hearingNormal blood clottingNormal pigmented skin

no dimplesattached earlobesno frecklesthin lips

Susceptibility to poison ivy

Congenital deafnessHemophiliaalbinism

* sex-linked characteristic

Dominant-Recessive Genes

• Simplest set of genetic rules = dominant-recessive pattern

• a single dominant gene strongly influences phenotype

• Because inherit one chromosome from each parent, our genetic instructions either=

• Homozygous (the same)• Heterozygous (different)

– Homozygous for curly hair if received a gene from each parent for curly hair.

– Heterozygous if received a gene for curly hair from mom and a gene for straight hair from dad.

Polygenic Inheritance

• Many genes influence the phenotype.• controlled by more than one gene, ie.

height, weight, hair color, skin color (basically anything dealing with color).

Multi-Factorial Traits

• Physical traits influenced by both genes and environment.– Height is an example: If a child is ill,

poorly nourished, or emotionally neglected, s/he may be smaller than others her/his age.

– Psychological traits (intelligence, personality) influenced by both nature and nurture (multi-factorial)

Pregnancy

In the Beginning…

Nine months?

• Pregnancy begins when the zygote (male gamete + female gamete) implants itself in the lining of the woman’s uterus.

• Zygote send chemical messages for menstruation to stop.

Twenty weeks…•the uterus can be felt at the level of your bellybutton

•As a result, the pelvic colon and small intestines are crowded upward and backward.

The end!

• 40 weeks

• Common discomforts you may experience during the last weeks of pregnancy are frequent urination, increased constipation, edema (water retention), and aching of the legs

Prenatal Development

Developmental Principles:• Cephalocaudal

pattern – causes dev’t to proceed from the head down.

• Proximodistal pattern – causes dev’t to happen in an orderly way from the center of the body outward to the extremities.

Three Stages of Prenatal Dev’t:

1. Germinal Stage

2. Embryonic Stage

3. Fetal Stage

Germinal Stage

• First two weeks of gestation, from conception to implantation = germinal stage

• During first 24-36 hours after conception cell division begins

• Cells are alike until @ 4 days after conception; they form a hollow sphere with two layers = blastocyst.

• Placenta = organ that allows oxygen, nutrients and other substances to be transferred between mom and baby’s blood. (Brings mother’s and baby’s blood close to one another without allowing them to mix.)

• Blastocyst’s inner cells begin to specialize– Umbilical cord – connects

the embryo to the placenta

– Aminion – fluid-filled sac in which baby floats

Embryonic Stage

• Begins when implantation is complete (@ 2nd week)

• Ends at week 8• Embryo’s cells start

to specialize to form body’s organs (aka: organogenesis)

Fetal Stage

• Beginning at week 8 and ending at birth

• Weight increases from one-fourth ounce to 7-8 pounds

• Height increase from one inch to 20 inches

• Nervous system dev’t• Viability = baby’s ability

to live outside the womb (@ 22 weeks)

Gender Differences

• Male fetuses tend to be more physically active than female fetuses

• Male embryo secretes the male hormone testosterone

• Female fetuses tend to be more sensitive to external stimulation and advance more rapidly in skeletal dev’t

• Boys are more vulnerable to prenatal probs (spontaneous miscarriage; birth defects)

Prenatal Behavior

• 25th week, fetus responds to sounds and vibrations with heart rate changes, head turns and body movements.

• Neonates appear to remember stimuli to which they were exposed prenatally—their mother’s heartbeat, the odor of amniotic fluid…

• Very active fetuses tend to become children who are very active

• Fetuses who are less active than “average” are more likely to be developmentally delayed.

Genetic Disorders

• Autosomal Dx– Caused by genes located

on the autosomes.– i.e., phenylketonuria

(PKU) = involves a recessive gene that causes a baby to have prob’s digesting the amino acid phenylalanine. Can cause MR as the toxins build up in baby’s brain. More likely to occur in Caucusian babies.

– Sickle-cell disease = recessive dx that causes red blood cell deformities. Blood can’t carry enough oxygen to keep the body’s tissues healthy. More common in West African and African-American infants.

– Huntington’s disease = caused by dominant genes and not usually diagnosed until adulthood. Causes the brain to deteriorate and affects psychological and motor functions.

Genetic Disorders

• Sex-Linked Dx’s– Most sex-linked dx’s are

caused by recessive genes– i.e., red-green color

blindness = people have difficulty distinguishing between the colors red and green when they are next to each other.

– Hemophilia = the blood lacks the chemical components which cause the blood to clot.

– Fragile-X syndrome = person has a “fragile” or damaged spot on the X chromosome. Can cause MR that becomes progressively worse as children age.

– What if parents find out their baby has a genetic dx? Should this information influence their decisions?

Chromosomal Errors

• Chromosomal errors = Probs that occur when a child has too few or too many chromosomes

• Trisomies = a condition in which a child has three copies of a specific autosome. – Most common is trisomy

21 or Down syndrome (3 copies of chromosome 21)

• Sex-Chromosome Anomalies = anomalies associated with the sex chromosomes– Klinefelter’s syndrome –

XXY pattern. Affected boys usually look normal but have underdeveloped testes and a sparsity of sperm. At puberty, they experience both male and female changes.

Prenatal Influences on Development

• Environmental factors –– Teratogens –

environmental stimuli harmful to developing fetus

• Diseases during pregnancy – any disease that can cross the placenta – German measles = minor

effects on mother, major effects on fetus (blindness, deafness, heart disease)

– Genital herpes & AIDS= can cross placenta

• Drugs – legal & illegal– Caffeine = can slow

fetal growth & contribute to premature birth

– FAS = abnormally small head, irritability, hyperactivity & retarded motor development and cognitive development

Other Maternal Influences on Prenatal Development

• Diet– Inadequate nutrition may

result in premature birth and low birth weight

– Folic acid – need adequate amounts; inadequate amounts can result in diseases such as spina bifida

• Age– Societal shift = older

mothers– Older mothers = higher risk

for genetic abnormalities, miscarriage & stillbirth

– What about older fathers?

• Maternal Emotions– Studies show extreme

maternal stress is associated with low birth weight and premature births.

• Environmental Hazards– Lead, mercury, x-rays– Effects may include

mental disability, retarded growth, impaired memory and verbal skill, and leukemia.

Prenatal Diagnosis and Treatment

• Genetic Counseling– Helps to assess the chances of inherited disorders.

• Prenatal Diagnosis– Ultrasound is the use of soundwaves to produce a

picture of the fetus.– Amniocentesis allows the taking of a sample of the

amniotic fluid.– Chorionic Villus Sampling (CVS) is taken from the

placenta and can be done earlier than amniocentesis.

Prenatal Diagnosis and Treatment (cont.)

• Fetal Medicine– Administering medicine to the fetus.– Fetal Surgery to correct spina bifida and

circulatory problems.– Genetic Engineering involves replacing

defective genes with synthetic normal genes.

Birth: The Final Push

• Labor and Delivery • What are the different phases of labor

and delivery?• What are “natural” ways of coping with

the pain of childbirth? Is childbirth at home safe?

• What are some complications that can occur during birth?

Labor and Delivery

• Stages of Labor• Stage 1 lasts 12-24 hours for the first

birth and includes contractions and the enlargement of the cervix to approximately 10 centimeters.

• Stage 2 includes the actual birth of the baby and lasts about an hour.

• Stage 3 lasts a few minutes and involves expelling of the placenta.

Labor and Delivery

• Approaches to Childbirth• Childbirth Classes

– Explains what happens during pregnancy and delivery.

– Teaches techniques to manage the pain of childbirth. The emphasis is on natural methods, relaxation, and coaching.

– Studies show that mothers who attend childbirth classes typically use less medication.

Labor and Delivery

• Approaches to Childbirth• Birthing Alternatives

– Home Births• Less Expensive• Parents have more control over the conditions• Birth problems are no more common at home than at

the hospital when pregnancy has been problem-free.

– Birthing Centers• More home-like than hospitals, but in a clinic setting

independent of hospitals.

Labor and Delivery: Birth Complications

• Hypoxia, or inadequate blood and oxygen to baby.

• Complications may result in cesarean section (C-section)

• Births before the 36th week are called premature or preterm.

• Babies weighing less than 5.5 pounds have low birth weight.

• Babies weighing less than 3.3 pounds have very low birth weight.

• Below 2.2 pounds is called extremely low birth weight.

Labor and Delivery

• Infant Mortality• Infant Mortality is the number of infants out of

1,000 births that die before the age of 1 year.• U.S. mortality rate is about 1%, or 9 of 1000.• 15 industrialized nations have lower infant

mortality than U.S.• Possible factors include low birth weight resulting

from a lack of free or inexpensive prenatal care and fewer paid leaves of absence for pregnant women.

Apgar TestSign 0 1 2Heart Rate Absent <100

beats/min100-140 beat/min

RespiratoryEffort

No breathing for 1 min

Slow and irregular

Good respiration normal crying

Muscle Tone

Limb and flaccid

Some flexion of extremities

Good flexion and active motion

Reflex Irritability

No facial expressions

Some contortions and grimacing

Active crying

Body color Blue or pale body/Extremities

Body pink w/blue extremities

Pink all over