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Chapter 3
Prenatal Development
What is the course of prenatal development?
Germinal Period—The period of prenatal development that takes place in the first
two weeks after conception and includes creation of the zygote, cell division, and
the attachment of the zygote to the uterine wall. After about a week, the blastocyst
consists of an inner layer of cells that will develop into the embryo and the
trophoblast, an outer layer of cells that later provides nutrition and support for the
embryo.
Embryonic Period—The period of prenatal development that occurs from two to
eight weeks after conception in which cell differentiation intensifies, support
systems for cells form, and organs appear. The embryo consists of three layers of
cells—the endoderm, or inner layer; the ectoderm, or outermost layer; and the
mesoderm, the middle layer.
Endoderm: inner layer of cells form digestive and respiratory systems
Ectoderm: outermost layer of cells form skin parts, nervous system, and sensory
receptors.
Mesoderm: middle layer of cells form bones, circulatory system, muscles,
excretory and reproductive systems
Every body part eventually develops from these three layers. The endoderm
primarily produces internal body parts, the mesoderm primarily produces parts
that surround the internal areas, and the ectoderm primarily produces surface
parts.
The amnion, umbilical cord, and placenta provide support and protection to the
embryo.
Placenta: consists of a disk-shaped group of tissues in which small blood vessels
from the mother and the offspring intertwine but do not join.
Umbilical cord: contains two arteries and one vein, and connects the baby to the
placenta
Amnion is like a bag or an envelope and contains a clear fluid in which the
developing embryo floats and amniotic fluid provides the baby an environment that
is temperature and humidity controlled, as well as shockproof.
The embryo’s developing organs are especially vulnerable to environmental
changes during organogenesis. By the time most women know they are pregnant,
the major organs have begun to form. Organogenesis is the name given to the
process of organ formation during the first two months of prenatal development.
Fetal Period—The period of prenatal development that begins two months after
conception and lasts for about seven months until birth. The fetus continues to
undergo dramatic growth and development during this period.
o Three months after conception, the fetus is about 3 inches long and weighs about
3 ounces. It has become active, moving its arms and legs, opening and closing
its mouth, and moving its head.
o By the end of the fourth month of pregnancy, the fetus has grown to 6 inches in
length and weighs 4 to 7 ounces. For the first time, the mother can feel arm and
leg movements.
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o By the end of the fifth month, the fetus is about 12 inches long and weighs close
to a pound. Structures of the skin have formed—toenails and fingernails.
o By the end of the sixth month, the fetus is about 14 inches long and has gained
another half-pound to a pound. As early as six months of pregnancy (about 24
to 25 weeks after conception), the fetus for the first time has a chance of
surviving outside the womb—that is, it is viable.
o By the end of the seventh month, the fetus is about 16 inches long and weighs
about 3 pounds.
The largest prenatal size and weight gains occur during the fetal period. Fingers,
toes, skin, features, lungs, other structures, and reflexes all develop and prepare for
birth. The three trimesters are not the same as the three prenatal periods. The
germinal and embryonic periods occur in the first trimester. The fetal period begins
toward the end of the first trimester and continues through the second and third
trimesters.
The Brain—By the time babies are born, they have approximately 100 billion
neurons, or nerve cells, which handle information processing at the cellular level in
the brain. As the human embryo develops inside its mother’s womb, the nervous
system begins forming as a long, hollow tube located on the embryo’s back. This
pear-shaped neural tube, which forms about 18 to 24 days after conception,
develops out of the ectoderm. Two birth defects related to a failure of the neural
tube to close are anencephaly and spina bifida. Once the neural tube has closed a
massive proliferation of new immature neurons begins to take place about the fifth
prenatal week and continues throughout the remainder of the prenatal period. The
generation of new neurons is called neurogenesis. At the peak of neurogenesis, it is
estimated that as many as 200,000 neurons are being generated every minute. The
basic structure of the brain develops during the first two trimesters of prenatal
development, while the third trimester is characterized by connectivity and
functioning of neurons.
What are Expectant Parents’ Experiences like During Prenatal Development?
Confirming the Pregnancy and Calculating the Due Date—fetal life begins with
the fertilization of the ovum, with pregnancy length calculated from the first day
of the woman’s last menstrual period. Usually lasts about 280 days or 40 weeks.
According to your text fetal life begins at ovum fertilization.
The Expectant Mother’s Nutrition, Weight Gain, and Exercise—Maternal weight
gains that average from 25 to 35 pounds are associated with the best outcomes;
the pattern of weight gain is also important. The ideal pattern of weight gain
during pregnancy is 2 to 4.4 pounds during the first trimester, followed by an
average gain of 1 pound per week during the last two trimesters. In the second
trimester, most of the weight gain is due to increased blood volume. In the third
trimester, weight gain mainly involves the fetus, placenta and amniotic fluid. The
recommended daily allowance for all nutrients increases during pregnancy and
water is essential.
Exercise during pregnancy can provide positive physical and mental outcomes.
o Changes in the women’s body: Some of the changes in the woman’s body
during pregnancy require adaptations in exercise.
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o Getting started: Before starting an exercise program, pregnant women
should talk with their doctor to ensure that they don’t have an obstetric or
health condition that might limit their activity.
o Choosing safe exercise: Most types of exercise are safe during pregnancy
but some types involve positions and movements that may be
uncomfortable, tiring, or harmful.
o Establishing a routine: Exercise is most practical during the first 24 weeks
of pregnancy; during the last three months many exercises that were easy
earlier in pregnancy are now often difficult. It is important to begin each
exercise session with a 5- to 10-minute warm-up and end each session
with a 5- to 10-minute cool down. During the warm-up and cool down,
stretching exercises should be done.
Prenatal Care—Prenatal care usually involves a defined schedule of visits for
medical care and comprehensive educational, social, and nutritional services.
Although research supports the significance of prenatal care to mothers and
infants across cultures, there are differences between ethnic groups in the United
States in terms of access to adequate resources. Rates of infant mortality and low
birth weight indicate that many other nations have healthier babies than the
United States.
Other countries outside the U.S.:
o Have lower rate of low-birth-weight infants
o Receive free or low-cost prenatal and postnatal care
o Enjoy liberal paid maternity leave
The Three Trimesters and Preparation for the Birth—First: The initial three
months when prenatal organ systems are formed; also a time of physical and
emotional adjustment for the mother. Second: The middle months of pregnancy,
marked by continued growth of fetus and the mother’s physical adjustment to the
change. Third: In the last three months of pregnancy, crowding by the uterus, in
addition to high levels of progesterone, may cause the mother to experience
heartburn, indigestion, shortness of breath, and other physical discomforts.
Preparation for the Baby’s Birth—About two weeks before the baby’s birth, the
fetus descends into the pelvic cavity and uterine contractions increase.
Awkwardness and fatigue may add to the mother’s desire for the pregnancy to
end.
Cultural Beliefs about Pregnancy and Development—Health-care practices
during pregnancy are influenced by the prevalence of traditional home care
remedies and folk beliefs, the importance of indigenous healers, religious beliefs,
and the influence of professional health-care workers. For example,
o Food cravings. Latin American, Asian, and some African cultures believe
that it is important for a pregnant woman’s food craving to be satisfied
because they are thought to be the cravings of the baby.
o “Hot-cold” theory of illness. Many cultures in Latin America, Asia, and
Africa characterize foods, medications and illnesses as “hot” or “cold”;
this has nothing to do with temperature or spiciness, but with traditional
definitions and categories.
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o Extended family. In many immigrant cultures, the extended family is a
vital support system, and health-care decisions are made based on the
needs of the family over those of the individual
o Stoicism. In many Asian cultures, stoicism is valued, as suffering is seen
as part of life. Physicians are also viewed with great respect.
What are Some Potential Hazards to Prenatal Development
Some General Principles—A teratogen is any agent that can potentially cause a
physical birth defect. The dose, genetic susceptibility, and the time of exposure to
a particular agent influence the severity of the damage and the type of defect that
occurs. Teratogens include drugs, incompatible blood types, environmental
pollutants, infectious diseases, nutritional deficiencies, maternal stress, advanced
maternal and paternal age, and environmental pollutants.
Prescription and Nonprescription Drugs—Prescription drugs that can function as
teratogens include antibiotics, some antidepressants, and hormones;
nonprescription drugs that can be harmful include diet pills and aspirin. All drugs
(prescribed, illegal) can have effects on the unborn fetus.
Psychoactive Drugs—Drugs that act on the nervous system to alter states of
consciousness, modify perceptions, and change moods. The potential negative
effects of such drugs as alcohol, nicotine, and illegal drugs on the fetus are welldocumented; however the extent of the risk and harm varies.
Caffeine—A review of studies on caffeine consumption during pregnancy
concluded that a small increase in the risks for spontaneous abortion and low birth
weight occurs for pregnant women consuming more than 150 milligrams of
caffeine. A recent study revealed that pregnant women who consumed 200 or
more milligrams of caffeine a day had an increased risk of miscarriage.
Alcohol—Heavy drinking by pregnant women can be devastating to the offspring.
Fetal alcohol spectrum disorders (FASD) are a cluster of abnormalities and
problems that appear in the offspring of mothers who drink alcohol heavily during
pregnancy. The abnormalities include facial deformities and defective limbs, face
and heart.
Cocaine—Despite cautions, the weight of research evidence indicates that
children born to mothers who use cocaine are likely to have neurological and
cognitive deficits.
Methamphetamine—Methamphetamine, like cocaine, is a stimulant, speeding up
an individual’s nervous system. Babies born to mothers who use
methamphetamine or “meth,” during pregnancy are at risk for a number of
problems, including high infant mortality, low birth weight, and developmental
and behavioral problems.
Marijuana—Marijuana use by pregnant women is related to deficits in memory
and information processing in their offspring.
Heroin—It is well documented that infants whose mothers are addicted to heroin
show several behavioral difficulties at birth. The difficulties include withdrawal
symptoms, such as tremors, irritability, abnormal crying, disturbed sleep, and
impaired motor control.
Incompatible Blood Types—The first Rh-positive baby of an Rh-negative mother
is not usually at risk, but the risk increases with each subsequent pregnancy. A
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vaccine may be given to the mother within three days of the first child’s birth to
prevent her body from making antibodies that will attack Rh-positive fetuses in
future pregnancies.
Environmental Hazards—Many aspects of the modern industrial world can cause
chromosomal abnormalities. Exposure to toxins, radiation, chemical pollutants,
and excess heat can all be hazardous to the embryo or fetus.
Maternal Diseases—Diseases and infections such as rubella, syphilis, genital
herpes, and AIDS can produce defects in the embryo or fetus by crossing the
placental barrier or causing damage during birth.
Other Parental Factors such as maternal diet and nutrition, maternal age, emotional stress,
and paternal health and age have varying degrees of impact on the fetus.
• Maternal diet and nutrition
– Folic acid and iron-Lack of folic acid is linked with neural tube defects in
offspring, such as spina bifida. Recommended dosage is a minimum of
400 milligrams per day, and good examples would include orange juice
and spinach
– Fish: PCBs and mercury levels- Some fish contain high levels of mercury.
Mercury is easily transferred across the placenta, and the embryo’s
developing brain and nervous system are highly sensitive. PCB’s
(polychlorinated biphenyls) are chemicals that were used in
manufacturing.
• Maternal age
– Highest risks: adolescents, over 35 years
• Maternal emotional states
Other Paternal Factors
• Father’s diet and low vitamin C can contribute to a higher risk of birth defects and
cancer.
• Drug use effects on sperm can contribute to a lower sperm count
• Smoking during pregnancy – effects of second-hand smoke
• Father’s age at conception
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Prenatal Development