Page 66-106, 42 pages Page 1 of 11
Chapter 3: Prenatal Development, Birth, and the Newborn
FROM CONCEPTION TO BIRTH
• Early researchers thought the sperm contained a miniature man, or homunculus which was pre-
formed and just needed to grow in the woman’s womb
• Germ cells produced through meiosis each have 23 chromosomes; men make sperm continuously
while women are born with all the eggs she will ever have (eggs will start to deteriorate with age)
• Prenatal Development consists of the changes that transform a fertilized egg into a newborn human,
taking an average of 38 weeks divided into three stages.
Period of the Zygote (Weeks 1-2)
• On average 500 million sperm are released, and after 6 hours only 200 get close to the egg. Some fail
due to chance, while others fail due to genetic defects
• Fertilized egg is called a zygote. Major events between fertilization to implantation:
o Fertilization takes place in upper third of fallopian tube, within 24 hours after ovulation
o 24-30 hours after fertilization, sperm and egg chromosome material unite
o Egg divides for first time – 2 cells at 36 hours; 4 cells at 48 hours
o On 3 day, cluster of 16-32 cells
o On 4 day, hollow ball of about 100 cells – blastocyst, less than 1mm in diameter
o 4-5 days zygote enters the uterus
o 6-7 days zygote begins to attach to inner wall of uterus
o 12-14 days zygote completes implantation in a week, burrowing into the uterine wall and
establishing connections with the mother’s blood vessels. This triggers hormonal changes in the
• The zygote may separate into two clusters that develop into identical twins (or even more – Dionne
quintuplets in 1930s); fraternal twins arise from two eggs being released, each fertilized by a different
• The blastocyst cells begin to differentiate. The germ disc develops into the fetus, while the other cells
become structures that support, nourish, and protect the developing organism; the layer closest to the
uterus becomes the placenta, a structure for exchanging nutrients and wastes between the mother and
• Factors responsible for fraternal twinning: ethnicity (highest among African-Americans), family history of
twinning, age (rises with maternal age), nutrition (tall and stronger build women), number of births (more
likely with each additional birth), use of fertility drugs and in vitro fertilization Page 66-106, 42 pages Page 2 of11
Period of the Embryo (Weeks 3-8)
• Once a blastocyst is completely embedded in the uterine wall it is called an embryo, from the 3 weekrd
after conception to the end of the 8 week.
• The period of the embryo is marked by major development in all organs and systems of the body,
occurring through cell division, cell migration, cell differentiation, cell death (apoptosis of tail, webbing
between fingers), and hormonal influences
• Development proceeds in cephalocaudal (head to tail) and proximodistal (inside to outside) manner
• The outer layer ectoderm becomes hair, nails, eyes, and the nervous system. The mesoderm forms
muscles, bones, and the circulatory system. The inner endoderm forms the digestive system, lungs, and
• 4 weeks: Flat set of cells in ectoderm curl to form the U-shaped neural tube, with one end swelling to
form the brain and the rest the spinal cord. Heart beating but otherwise little cell differentiation.
• 5-6 weeks: Rapid facial and brain development, starting to move.
• 8 weeks: The embryo seems to have an eye, jaw, arm, leg, and developing brain and heart.
• The embryo rests in an amniotic sac filled with amniotic fluid that cushions the embryo and maintains
a constant temperature; fluid also allows embryo movement.
• The umbilical cord houses blood vessels that join the embryo to the placenta; in the placenta, the
mother’s blood vessels and the umbilical blood vessels exchange nutrients, oxygen, and waste products
through finger-like villi projections
instead of mixing directly
Period of the Fetus (Weeks 9-38)
• The period of the fetus is the final
and longest phase of prenatal
development, from 9 week after
conception to birth.
• Continued development of
physical structures, rapid growth,
increasing levels of behaviour,
sensory experience, and learning
• There is remarkable weight gain,
from a few grams to 110-225g at four months, then another 3-4kg at birth.
• 9 weeks: By start of fetal period, brain has distinct structures and has begun to regulate body functions.
All regions of the brain grow, particularly the cerebral cortex.
• 11-12 weeks: Testes secrete hormones that cause the development of a penis and scrotum; in a
female, this hormone is absent, so the same cells become a vagina and labia. Fingers also fully formed;
reflexes present; organs, muscles, and nervous system become more organized.
• 16 weeks: Mom begins to feel babies’ first kicks. Beginning of period of lower body growth.
• 18 weeks: Starts sucking thumb. The skin thickens and is covered with vernix, a thick greasy
substance that protects the fetus.
• 20 weeks: More head-down position and less movement. Begins putting on more weight. Facial
expressions begin; reflexes including shielding eyes from fetoscope. May survive at 20-22 weeks with a
lot of medical intervention – but lack temperature regulation, proper immune system, and developed
• 5 -6 months: Eyebrows, eyelashes, and scalp hair emerge. Page 66-106, 42 pages Page 3 of 11
• 28 weeks is the age of viability, most systems including brain function well enough that a fetus born at
this time has a chance to survive of born prematurely. Eyes are open and moving, and auditory system
• However, they will likely have trouble breathing and cannot regulate body temperature properly as they
lack the insulating layer of fat that appears in the 8 month – in the last 3 months, fetus triples in weight.
• Normal formation of organs and muscles depend on fetal activity – by 12 weeks will have all newborn
• The fetus starts to move more noticeably at regular activity cycles; when active, once a minute they
may punch, kick, and somersault. They experience tactile stimulation with hands touching walls of the
• An active fetus is more likely than an inactive fetus to be an unhappy, difficult baby with individual
differences in activity continuing to postnatal life
• The fetus can hear and respond to sounds such as the mother’s heartbeat and speech, will often prefer
mom’s voice after birth.
• The fetus can also taste, swallowing the amniotic fluid – this helps palate development.
• Vision is the least developed sense, as very little light filters through the womb
• Chest movements similar to breathing occur, though not constantly
• Habituation studies done in utero
and tested after birth demonstrate
fetal learning in many senses
• When read Dr. Seuss prenatally, will
identify cadence, intonation and
stress pattern after birth by reacting
with attention (measured with
movement and decelerated heart
• Tastes from pregnancy affect tastes of the baby – women who drank carrot juice several times a week
during last month of pregnancy had 5-6 month old infants who preferred carrot juice.
• Keep head turned towards own amniotic fluid, prefer mom’s voice, prefer own language
Tstmester Events Prenatal Care Serious Complications
1 Trimester: First day Missed period Confirmation of pregnancy Ectopic pregnancy:
of last menstrual Breast enlargement Calculation of due date implantation in oviducts
period to 12 weeks Abdominal thickening Blood and urine tests Abnormal urine or blood tests
Monthly doctor visits to monitor vital Increased blood pressure: pre-
functions, uterine growth, weight gain eclampsia possible
2 Trimester: 12 Weight gain Monthly doctor visits Gestational diabetes
weeks to 24 weeks “Showing” Ultrasound to measure fetal growth Excessive weight gain
Fetal movements felt and locate placenta Increased BP
Increased appetite Rh incompatibility
3 Trimester: 25 Weight gain Weekly visits from 32 weeks Increased BP
weeks to beginning of Breast discharge Ultrasound to assess position of fetus Premature labour
labour Treatment of Rh compatibility if Bladder infection
Pelvic exams for cervical dilation Page 66-106, 42 pages Page 4 of 11
INFLUENCES ON PRENATAL DEVELOPMENT
• 45% of zygotes spontaneously abort due to severe defects, before pregnancy is known
• 15-20% of known pregnancies miscarry
General Risk Factors
• The mother is the developing child’s sole source of nutrition. She needs a balanced diet, 10-20%
increase in caloric intake, and will likely gain 11-16kg during pregnancy (1/3 from baby, placenta, and
amniotic fluid, 1/3 from increased fat stores, 1/3 from increased volume of blood, breasts, and uterus).
• Folic acid, a B vitamin, is important for nervous system development. Without adequate amounts,
babies are at risk of spina bifada, or “cleft spine” where the neural tube does not close properly in the
• This results in permanent damage to the spinal cord and nervous system. Incidence has significantly
reduced since food fortification regulations were enacted.
• Dutch Famine Study: Those conceived in the Holland famine situation during WWII rations of 1940
were more likely to be born with neurodevelopmental abnormalities and increased risk of disorders, such
• Incidence of infant mortality especialrd high
for mothers who experienced famine in 3
• Stress is a person’s physical and
psychological response to threatening or
challenging situations. In animal
experiments, this leads to smaller offspring
more prone to physical and behavioural
• Women who report greater anxiety during
pregnancy often give birth early or have
underweight babies; their children may also
have attention and behavioural problems.
• Stress hormones reduce the flow of oxygen to the fetus, while increasing heart rate and activity level.
Stress can also weaken the mother’s immune system, making her and the fetus susceptible to illness.
Lastly, pregnant women under stress are more likely to smoke and drink, and less likely to rest, exercise,
and eat properly.
• Teenagers: Compared to 20-somethings, teenagers are more likely to have problems during pregnancy,
labour, and delivery because most pregnant teenagers are more likely to be economically disadvantages
and do not receive good prenatal care.
• When differences in prenatal care are accounted for, teenagers are as likely to have problem-free
pregnancies and give birth to healthy babies. However, these children tend to do less well in school and
have more behavioural problems due to other factors; two distinct mechanisms are at work:
• Social Influence: Events set in motion when a teenage girl gives birth make it harder for her to provide
an environment positive for her child’s development, e.g. dropping out of school limits employment
• Social Selection: Factors that caused the teenage girl to become pregnant also put their children at
risk. E.g. Conduct disorder teenagers more likely to get pregnant, lie and break rules, and act aggressive
physically and verbally; in addition, conduct disorder has a genetic component which may be passed to
their children. Page 66-106, 42 pages Page 5 of11
• Social selection factors are independent of mother’s age of birth. Factors that put girls at risk for
becoming pregnant also puts their children at risk.
• Jaffee (2001): Children born to teenage mothers 3x more likely to have committed criminal offence.
Social influence: teenage mothers had lower income, predicted children’s criminal activity. Social
selection: teenage mother history of antisocial behaviour, predicted children’s criminal activity.
• Older Women: Older women have more difficulty getting pregnant, and are less likely to have
successful pregnancies with risks of miscarriage, stillbirth, and abnormalities like Down syndrome much
• Preeclampsia is a serious disorder involving high blood pressure, sudden weight gains, headaches,
and edema of the face, hands, and feet. Without immediate treatment, may progress rapidly; it is the
leading cause of maternal infant illness and death. Risk of preeclampsia greatly increases after age 40.
• Paternal age is also involved in risk of congenital malformations due to mutations of gametes in older
men. Associated with higher risk of Down syndrome and Autism Spectrum disorders.
Teratogens: Diseases, Drugs, and Environmental Hazards
• Teratogen: Agent which causes abnormal prenatal development, may have mild effects or death
depending on dose and timing. Thalidomide originally used to reduce morning sickness, it caused limb
deformities in babies.
• Several bacterial and viral infections can be harmful to the embryo or fetus: AIDS, cytomegalovirus,
genital herpes, rubella (German measles), and syphilis.
• Some of these pass from the mother through the placenta to attack the embryo or fetus directly
(rubella, syphilis, cytomegalovirus), while others are transmitted at birth through the birth canal; AIDS is
transmitted both ways.
• Routinely-used substances like alcohol, aspirin, caffeine, and nicotine can harm the child.
• Nicotine constricts blood vessels, reducing oxygen and nutrients that can reach the fetus; it also
increases risk of early placenta separation and premature membrane rupture (water breaking).
• Pregnant women who smoke are more likely to miscarry, or give birth to lower weight babies. These
children are more likely to have impaired cognitive skills, attention, and behavioural problems. Second-
hand smoke for pregnant women is also a factor.
• Pregnant women who consume large amounts of alcohol can have babies with fetal alcohol
syndrome (FAS). Alcohol passes through the placenta, and concentration quickly equalizes in the baby
– however, lack liver enzymes for proper breakdown, use up additional oxygen; interferes with cell
duplication and migration
• FAS children tend to have heart problems, distinct facial irregularities (small head, thin upper lip, short
nose, widely spaced eyes); they also tend to experience developmental delays and cognitive and
behavioural problems, including ADHD.
• Even moderate drinking can lead to a lower-level set of symptoms, Fetal Alcohol Effect (FAE) with
deficits in attention, memory, and intelligence.
• FAS may be diagnosed by an eye movement test, with FAS children showing a specific pattern of
movement not found in other conditions. Typically physical characteristics and intellectual + behavioural
deficits are used
• Illegal drugs like cocaine are less studied due to lack of data. Cocaine is known to cause premature
birth, low growth, cognitive defects, and inability to regulate arousal and attention.
• Toxin exposure from foods, air, and environment may cause damage to a developing fetus. Even before
conception, environmental chemicals can damage gamete DNA. On the other hand, spontaneous
mutagenesis may contribute positively to genetic diversity. Page 66-106, 42 pages Page 6 of11
• Pollutants like polychlorinated biphenyls (PCBs) or lead can cause cognitive deficits (IQ, memory,
verbal skill), or disorders like schizophrenia.
• X-rays cause retarded growth, intellectual disabilities, and leukemia. However, electromagnetic
radiation associated with computer monitors and other video displays have been found to have no
• Ratio of male children born in the Aamjiwnaang First Nation has declined to 1/3 from 1984 to 2003.
Levels of chemical pollutants including dioxins and PCBs from the nearby Chemical Valley in Sarnia
found to be correlated with decline in male birth radio.
• Pregnant women should avoid convenience foods with chemical additives, air contaminated with
cleansers, paint, and fertilizers, and wear protection to reduce contact with potential teratogens.
• Occupational hazards: toll booth workers (automobile exhaust), farmers (pesticides), factory workers
How Teratogens Influence Prenatal Development
• Impact depends on genotype of organism: A substance may be harmful to one sp