Prenatal Development and the Newborn Period
• Results from the union of two gametes, the egg and the sperm
• Gametes are produced through a specialized cell division, which results in
each gamete’s having only half the genetic material of all other normal cells in
Female Reproductive System
• The process of reproduction starts with the launching of an egg from one of
the woman’s ovaries into the fallopian tube.
• If sperm are available near the time the egg is released, then conception will
Sex Differences Begin at Conception
• Approximately 120 to 150 males are conceived for every 100 females because
the sperm bearing the Y chromosome are lighter and swim faster to the egg
than those bearing an X chromosome.
• However, male embryos are miscarried at higher rates than female embryos,
and boys are more vulnerable to developmental disorders
• Males are also more vulnerable to illness throughout the life span
• The fertilized egg, or zygote, has a full complement of human genetic
material, half from each parent.
• Marks the beginning of the three periods of prenatal development
• Germinal (conception to two weeks)
• Embryonic (3 to 8 week)
• Fetal (9 week to birth)
B. Developmental Processes
• Four major developmental processes transform a zygote into an embryo and
then into a fetus.
1. Cell division results in the proliferation of cells
2. Cell migration is the movement of cells from their point of origin to
somewhere else in the embryo
3. Cell differentiation transforms the embryo’s unspecialized stem cells
into different types of cells
4. Apoptosis, genetically programed cell death, also enables prenatal
• The idea that because of our common evolutionary history, humans share
some characteristics and developmental processes with other animals,
The Role of Hormones • Hormones play a crucial role in sexual differentiation.
o All humans’fetuses can develop either male or female genitalia,
depending on the presence or absence of testosterone.
• One of the many ways in which the fetus acts as an instigator of its own
C. Early Development
• By the 4th day after conception, the zygote arranges itself into a hollow sphere
of cells with a bulge of cells, the inner cell mass, on one side.
o The inner cell mass eventually forms into the embryo
• Identical twins originate from the splitting in half of the inner cell mass,
resulting in the development of genetically identical individuals
• Fraternal twins result when two eggs are released into the fallopian tube at the
same time and are fertilized by different sperm
• After implantation, the inner cell mass becomes the embryo and the rest of the
cells develop into its support system.
• The neural tube is a U-shaped groove formed from the top layer of
differentiated cells in the embryo
o It eventually becomes the brain and the spinal cord.
• The support system includes:
• Placenta: permits the exchange of materials between the bloodstream of the
fetus and that of the mother
• Umbilical cord: the tube that contains the blood vessels that travel from the
placenta to the developing organism and back again
Protecting the Fetus
• The placental membrane is a barrier against some, but not all toxins and
• The amniotic sac, a membrane filled with fluid in which the fetus floats,
provides a protective buffer for the fetus
D. Fetal Behavior
• By 12 weeks after gestation, most of the movements that will be present at
birth have appeared.
• Prenatal to postnatal continuity
• Swallowing amniotic fluid promotes the normal development of the palate and
aids in the maturation of the digestive system
• Movement of the chest wall and pulling in and expelling small amounts of
amniotic fluid help the respiratory system become functional
• Become stable during the second half of pregnancy
• Circadian rhythms are also apparent • Near the end of pregnancy, the fetus’sleep and wake states are similar to those
of the newborn
E. Fetal Experience
• The sensory structures are present relatively early in prenatal development and
play a vital role in fetal development and learning.
o Prenatal visual experience is negligible
o Fetus experiences tactile stimulation as a result of its own activity, and
tastes and smells the amniotic fluid th
o It responds to sounds from at least the 6 month of gestation
G. Fetal Learning
• At 32 weeks gestation, the fetus decreases responses to repeated or continued
stimulation, a simple form of learning called habituation.
• Newborn infants have been shown to recognize rhymes and stories presented
• Newborns also prefer smells, tastes, and sound patterns that are familiar
because of prenatal exposure
H. Hazards to Prenatal Development
• So far the focus has been on the normal course of prenatal development, but
sometimes there are hazards that occur during prenatal development.
• By far the most common misfortune in prenatal development is spontaneous
• Around 45% or more of conceptions result in very early miscarriages, and
about 15-20% of pregnancies of which women are aware are miscarried
• The majority of embryos that miscarry very early have severe defects
• Teratogens are environmental agents that have the potential to cause harm
during prenatal development.
• Timing is a crucial factor in the severity of the effects of potentially harmful
o Many agents cause damage only if exposure occurs during a sensitive
period in development
• Most teratogens show a dose-response relation.
o Increases in exposure to potential teratogens are associated with
greater probabilities of fetal defects and with more severe problems.
• Individual differences also influence the effects of teratogens.
• Identifying teratogens is made difficult by the existence of sleeper effects in
which the impact of a given agent may not be apparent for many years
• Teratogens include legal as well as illegal substances...
• Cigarettes o Cigarette smoking during pregnancy is linked to retarded growth and
low birth weight.
o Cigarette smoking has also been linked to SIDS (sudden infant death
syndrome), although the ultimate causes of SIDS are still unknown
o Parents can reduce the risk of SIDS by not smoking, putting babies to
sleep on their backs rather than on their stomachs, using firm
mattresses and no pillows as bedding for infants, and avoiding
wrapping infants in lot