CHAPTER 3

LO1 Describe the key events of the germinal stage.
LO2 Describe the key events of the embryonic stage.
LO3 Describe the key events of the fetal stage.
LO4 Describe environmental influences on prenatal development, including maternal nutrition, teratogens, infections, parental drug use, environmental hazards, and parents’ age.
© Tetra Images/Jupiterimages

• T F
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Newly fertilized egg cells survive without any nourishment from the mother for more than a week.
Your heart started beating when you were only one-forth of an inch long and weighed a fraction of an ounce.
Parents in wealthy nations have more children.
The same disease organism or chemical agent that can do serious damage to a 6week-old embryo may have no effect on a 4month-old fetus.
It is harmless to the embryo and fetus for a pregnant woman to have a couple of glasses of wine in the evening.

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• Within 9 months
– A fetus develops from microscopic cell to a neonate about 20 inches long.
• Normal gestation periods
– 280 days from onset of last menstrual period before conception
– 266 days from assumed date of fertilization (2 weeks after the beginning of last menstrual cycle)
• Stages of gestation
– Germinal: first 2 weeks
– Embryonic: 3rd - 8th week (2 months)
– Fetal: 3rd month to birth
– Also commonly used is TRIMESTER system
• 3 trimesters = 3 months each

• The period from conception to implantation
– 36 hours from conception zygote divides into 2 cells
– Continues dividing on 3-4 day journey to uterus
– By 72 hours becomes 32 cells
– Mass of dividing cells wanders about the uterus until finally implanting in uterine wall
– Total time approx. 2 weeks

Figure 3.1 – The Ovarian Cycle, Conception, and the Early Days of the Germinal Stage
Division of the zygote creates the hollow sphere of cells termed the blastocyst, which becomes implanted in the uterine wall.

(1st 2 weeks)
• Terms and concepts:
– Blastocyst
• Fluid filled ball of dividing cells that begin to separate into groups and eventually become different structures
– Embryonic disk
• Plate-like inner part of blastocyst that becomes the embryo
& fetus
– Trophoblast
• Outer part of blastocyst that becomes 4 different membranes
– 1 - makes blood cells until liver develops
– 2 - develops the umbilical cord and placenta
– 3 - develops into amniotic sac
– 4 - becomes the chorion that lines the placenta

(1st 2 weeks)
• Terms and concepts, con’t.
– Umbilical Cord
• The tube that connects the developing fetus to the placenta
– Placenta
• Organ connected to uterine wall and fetus by the umbilical cord. Serves as relay station between mother and fetus for exchange of nutrients and wastes.
– Implantation Bleeding
• May be caused when small blood vessels lining the uterus are ruptured during implanting process
• Most women experiencing this have normal pregnancy
– Miscarriage
• Usually stems from abnormalities in development process
• Nearly 1/3 of pregnancies end in miscarriages, most within first 3 months

© Tetra Images/Jupiterimages

• The embryonic stage begins with implantation and covers the first 2 months.
• Characterized by major organ development following:
– Cephalocaudal trend:
• From head to tail
– Proximodistal trend:
• From near to far
– Early brain and organs near spine allows them to play key roles in further development
• Growth of head takes precedence
• Organ systems near spine grow prior to the extremities

Figure 3.2 – A Human Embryo at 7 Weeks
Note that the head is oversized in relation to the rest of the body.
© Petit Format/Nestle/Science Source/Photo Researchers

(3-8 weeks)
• Terms and concepts:
– Ectoderm
• Outer layer of embryonic disk cells develop into:
– Nervous system, sensory organs, nails, hair, teeth, outer layer of skin
– Neural tube
• Two ridges in the blastocyst fold and form a tube
– This will become the nervous system.
– Endoderm
• Inner layer of the embryo develops into:
– Digestive and respiratory systems, liver, pancreas
– Mesoderm
• Middle layer of embryo develops into:
– Excretory, reproductive, and circulatory systems; and the muscles, skeleton, and inner layer of skin

The Embryonic Stage
(3-6 weeks)
• Time frame developments:
– 3rd week:
• Head and blood vessels begin to form
– 4th week:
• Arm and leg buds begin to appear
• Eyes, ears, nose, and mouth start to shape
• Nervous system/brain begins developing
– 5-6 weeks:
• Embryo about 1/4 - 1/2 inch long
• Heart begins to beat
• Both internal and external genitals resemble female structures
– 6-8 weeks:
• Head rounded and distinct facial features appear
• 7th week genetic XX or XY codes causing sex organs to differentiate
• Hands and feet (initially with webbed toes and fingers, but separate by end of 8th week)
• Teeth buds form; kidneys begin filtering blood; liver starts to produce blood cells
• Embryo is approx. 1 inch long and weighs about 1.30th ounce

The Embryonic Stage (6-8 weeks)
• Time frame developments:
– 6-8 weeks:
• Head rounded and distinct facial features appear
• 7th week genetic XX or XY codes causing sex organs to differentiate
• Hands and feet (initially with webbed toes and fingers, but separate by end of 8th week)
• Teeth buds form; kidneys begin filtering blood; liver starts to produce blood cells
• Embryo is approx. 1 inch long and weighs about 1.30th ounce
© Comstock Images/Jupiterimages

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(9weeks/3mos - Birth)
• This final stage in gestation lasts from the start of the
3rd month (9 weeks) until birth
• 1st trimester - 9-12 wks (3 mos):
– Major organ systems formed
– Begins to turn and respond to external stimulation
– Fingers and toes fully formed
– Eyes and sex can be clearly seen

(9weeks/3mos - Birth)
• 2nd trimester - 13-24 wks (6 mos):
– Dramatic gains in size - from 1 oz. to 2 lbs. and 3 inches to 14 inches
– Opens and shuts eyes, sucks thumb, sleep/wakes, perceives light and sound
– Downy hair growth
• 3rd trimester - 25-36 wks (9 mos):
– Organs continue to mature
– Gains about 5.5 lbs and doubles in length
– At 7th month nearly doubles weight and grows another 2 inches
– Chances of survival if born now nearly 90% - at 8 mos. Odds overwhelmingly in favor
– Newborn boys avg. 7.5 lbs. Girls avg. 7 lbs.

• By week 13, fetus responds to sound.
• During 3rd trimester, fetus discriminates to differences in pitch.
– Exhibiting variety in movement and changes in heart rate
• DeCasper & Fifer experiment (1980)
– Newborns exhibit preference for readings of mother during final month and half of pregnancy
• Fetal learning may be one basis for developing attachment to mother.

• First fetal movements felt by mid-4th month
• 2/30 weeks fetus becomes very active
– Turning somersaults clearly felt by mother
– No danger of umbilical cord breaking or wrapping around fetus at this time
• Different patterns of prenatal activity
– 5-6 months
• Slow squirming movements
• Also sharp jabbing and kicking about same time
• Growth eventually causes fetus to become cramped for space
– Activity becomes markedly less active during the final month of gestation

• U.S. Census Bureau “guesstimates” pre-history populations
– 10,000 years ago: 5 million humans
– 5,000 years ago: 14 million
• Year 1
– 170 million
• 1900
– 1.7 billion
• 1950
– 2.5 billion
• Today
– Approx. 6.5 billion
• Population explosion relative to location on planet

© Tetra Images/Jupiterimages

• The developing fetus is subject to many environmental hazards:
– Nutrition
– Teratogens
– Critical Periods of Vulnerability
– Sexually Transmitted Infections
– Rubella
– Toxemia
– Rh Incompatibility
– Drugs Taken by the Parents
– Other unknown hazards
– Age of parents

• Maternal malnutrition
– Linked to low birth weight, premature birth, retardation, cognitive deficiencies, behavioral problems, and cardiovascular disease
– Misconception that fetus can “take what it needs”
– Effects of fetal malnutrition
• May sometimes be counteracted by a supportive, care-giving environment
• Enriched daycare programs show improved intellectual and social skills by age 5

• Maternal obesity
• Linked with higher risk of stillbirth and neural tube defects
• Typical weight gain
– Without restricting diet, normal weight gain is 25-35 lbs.
– .05 lbs. pr wk during 1st half of pregnancy & 1 lb. per wk. thereafter is desirable
– Sudden large gains or losses should be discussed with doctor

• Environmental agents that can harm the embryo or fetus:
– Drugs
• Alcohol and other drugs
– Substances produced by the mother’s body
• Rh-positive antibodies
– Heavy metals
• Lead and mercury
– Hormones
• Healthful in many ways, but harmful if in excess
– Radiation
• X-rays; nuclear bombs
– Disease-causing organisms (pathogens)
• Bacteria and viruses

• Exposure to certain teratogens is most harmful during
that correspond to times when certain organs are developing
• Major organs differentiate during
Embryonic Stage making an embryo generally more vulnerable than a fetus

Figure 3.3 – Critical Periods in Prenatal
Development

• Syphilis can cause:
– Miscarriage, stillbirth, congenital syphilis
(present at birth)
– Detected with early routine blood tests
– Treated with antibiotics
– Fetus will probably not contract if treated before 4th month of pregnancy
– If left untreated:
• Fetus may be infected in utero and develop congenital syphilis
• Approx. 12% of those infected die

• HIV/AIDS
– Disables body’s immune system leaving victims prey to a variety of fatal illnesses
– Lethal unless treated with antiviral drugs; treatment not guaranteed
– Transmitted by sexual relations, blood transfusions, sharing hypodermic needles, childbirth, breast feeding
– 1/4 of babies born to HIV-infected mothers are infected also
• Due to rupture of blood vessels during childbirth
• HIV also found in breast milk; transmission of HIV affects about 1-6 (16.2%)

• A viral infection
• 20% chance of birth defects if infected during first 20 weeks of pregnancy
– Deafness, mental retardation, heart disease, blindness
• Most adult women had rubella as children which offers immunity.
• Vaccination can be done during pregnancy but best done prior.
• Inoculations have resulted in dramatic decline of birth defects from Rubella.
– 1964-65 = 2,000 CASES
– 2001 = 21

• Life threatening disease related to maternal deaths
– High blood pressure in late 2nd or early 3rd trimester
– Often causes premature birth or undersize babies
– Linked to malnutrition but causes are unclear
– Women not receiving adequate prenatal care are more susceptible.

• Rh is a blood protein found in red blood cells of some individuals.
• Rh Incompatibility occurs when:
– Mother is Rh-neg and fetus is Rh-pos (can get pos. cells from father)
• The neg./pos. combo occurs in approx. 10% of
U.S. couples.
• If antibodies enter fetal bloodstream:
– Can cause anemia, mental deficiency, death
– Fetus or newborn at risk can receive a blood transfusion to remove the mother’s antibodies.

• All medications (even OTC meds like aspirin) can be potentially harmful to a fetus.
– Always seek medical advice before taking ANY meds.
• Some common drug problems:
– Thalidomide
– Hormones
- Caffeine
- Cigarettes
– Vitamins
– Heroin and Methadone
– Marijuana
– Cocaine
– Alcohol

• THALIDOMIDE
– Marketed in 1960s for treating insomnia and nausea; used to alleviate “morning sickness”
– Available in Germany and England without Rx
– Within a few years, more than 10,000 babies were born with severe birth defects.
• Missing and malformed/stunted arms and legs
– A dramatic example of critical periods to teratogens

• HORMONES
– Women at risk of miscarriage may be prescribed hormones to help maintain pregnancy.
– PROGESTIN
• Similar to male sex hormones; can masculinize external sex organs of female embryos
– DES
• Powerful estrogen
• Given to many women in 1940-50s to prevent miscarriages
• Causes cervical and testicular cancer in some offspring
• 1 in 1,000 daughters of DES users develop cancer in reproductive organs
• Also more likely to have premature and low birth weight babies
• Both daughters and sons of users have high rates of infertility and immune system disorders.

•
– Normal use of multivitamins for mothers pose no threat and are often prescribed.
– High doses of Vitamins A and D have however been associated with:
–CNS damage
–Small head size
–Heart defects

•
– Easily cross the placental membrane
– Fetuses of regular users can become addicted in utero.
– Linked to low birth weight, premature birth, and toxemia
– Treatment of addicted newborns includes:
• Administering drug shortly after birth to prevent withdrawal; then gradually weaned from drug
• May have behavioral effects - motor and language delays at 1 year of age

• MARIJUANA
– Risks for fetus and child
• Slow growth rate
• Babies exhibit tremors, startling (CNS impairment)
• Cognitive effects
– Mixed findings in studies; none conclusive
– Some suggest problems with learning and memory
– Also increase in hyperactivity, impulsivity, short attention spans, increase in delinquency and aggression
– Overall reflects impairment in functioning in school and conforming to social norms

•
– Increases risk of stillbirth, low birth weight, and birth defects
– Infants often excitable and irritable or lethargic and sleep disturbed
– Delays in cognitive development seen at 1 year
– Also show problems at later ages with lower receptive and expressive language skills

•
– Even moderate drinking can place a fetus at risk for serious birth defects and increase risk for miscarriages
– Heavy drinkers place fetus at risk for
FETAL ALCOHOL SYNDROME (FAS)
• PHYSICAL SYMPTOMS
– Smaller size and brains; distinct facial features include wide set eyes, flat nose, underdeveloped upper jaw, malformed limbs, poor coordination, heart problems
• PSYCHOLOGICAL CHARACTERISTICS
– Retardation, hyperactivity, distractibility, less verbal fluency, learning disabilities
• MALADAPTIVE BEHAVIORS
– Poor judgment, poor perception of social cues

Figure 3.4 – Fetal Alcohol Syndrome (FAS)
© George Steinmetz

• Unknown exposure to other environmental hazards: a.k.a. pollution
• HEAVY METALS
– Lead, Mercury, Zinc
– Can delay mental development at ages 1 & 2 years
• PCB’s (industrial product waste found in water)
– Smaller babies, poorer motor functioning, memory deficits
• RADIATION
– Damage to eyes, CNS, and skeleton
– Results of WWII bombing of Japan saw high incidence of birth defects and mental retardation
• Father’s exposure can also effect child

Age of Parents
• Ideal age for childbearing = 20s
– Teen Mothers
• Higher incident of infant mortality, low birth weight
• Not adequately mature physically for stress of preg.
• Less educated and less likely to get prenatal care
– Older Mothers
• Fertility declines gradually until mid-30s then accelerates; after mid-30s reproductive system does not function as efficiently
• After 30-40s see increase in stillborns and preterms
• With adequate care, risk is lessened
– Older Fathers: more likely to produce abnormal sperm