Chapter 2: Prenatal Development and the Newborn Period
- Aristotle rejected the idea of preformation in favor of what he termed epigenesis
- Epigenesis: the emergence of new structures and functions in the course of development
- Gametes (germ cells): reproductive cells- egg and sperm- that contain only half the genetic
material of all the other cells in the body
- Gametes are produced through cell division 46 chromosomes 23 pairs of chromosomes
- Conception: the union of an egg from the mother and a sperm from the father
- Zygote: a fertilized egg cell
Four Major Developmental Processes that Transform a Zygote into an Embryo, then a Fetus:
1. Cell division: the zygote divides into two equal parts
2. Cell migration: the movement of newly formed cells from their point of origin in the embryo
to somewhere else
3. Cell differentiation: cells start to specialize in terms of both structure and function
4. Death: those cells that selectively disappear from the hand plates
- Embryo: the name given to the developing organism from the 3 to 8 week of prenatal
- Fetus: the name given to the developing organism from the 9 week to birth
- Stem cells: embryonic cells, which can develop into any type of body cell.
- Phylogenetic continuity: the idea that because of our common evolutionary history, human
share many characteristics, behaviors, and developmental processes with other animals,
- Apoptosis: genetically programmed cell death
- Inner cell mass is a hollow sphere with a bulge of cells
- Identical twins: twins that result from the splitting in half of the zygote, resulting in each of the
two resulting zygotes having exactly the same set of genes.
- Fraternal twins: twins that result when two eggs happen to be released into the fallopian tube
at the same time and are fertilized by two different sperm; fraternal twins have only half their
genes in common.
- The amniotic sac and placenta enables the embryo to develop - Neural tube: a groove formed in the top layer of differentiated cells in the embryo that
eventually becomes the brain and the spinal cord
- Amniotic sac: a transparent, fluid-filled membrane that surrounds and protects the fetus
- Placenta: a support organ for the fetus; it keeps the circulatory systems of the fetus and mother
separate, but as a semipermeable membrane permits the exchange of some materials between
them (oxygen and nutrients from mother to fetus and carbon dioxide waste products from fetus
- Umbilical cord: a tube containing the blood vessels connecting the fetus and placenta
An Illustrated Summary of Prenatal Development
- The areas nearer the head develop earlier than those farther away (head before body, hands
- Cephalocaudal development: the pattern of growth in which areas near the head develop
earlier than areas farther from the head.
- The fetus is an active participant in, and contributor to, its own physical and behavioral
- From 5 or 6 weeks after conception, the fetus moves spontaneously, starting with a simple
bending of the head and spine that is soon followed by the onset of numerous kinds of
increasingly complex movements over the next weeks.
- Around 7 weeks the movement to emerge is hiccups
- By 12 weeks most of the movements that will be present at birth have appeared
- 19 weeks to 35 weeks thumb sucking
- Two important forms of fetal movement is swallowing and breathing
- Resting- activity cycles- bursts of high activity alternating with little or no activity for a few
minutes at a time- emerge as early as 10 weeks and become very stable during the second half
- The fetus experiences an abundance of sensory stimulation
Sight and Touch
- The fetus does experience tactile stimulation as a result of its own activity; its hands come into
contact with other parts of its body. Fetuses have been observed grasping their umbilical cords,
rubbing their faces, and sucking their thumbs. Taste
- The fetus has a sweet tooth and drinks more amniotic fluid when it had been sweetened.
- Amniotic fluid also takes on odors from what the mother has eaten
- Habituation: a simple form of learning that involved a decrease in response to repeated or
- Only if the infant remembers the stimulus from one presentation to the next can the stimulus
lose its novelty. When a new stimulus occurs, the habituated response recovers (increases).
- Newborns also have a natural preference for a familiar smell- the scent of the amniotic fluid in
which they lived for the previous 9 months.
- Newborns prefer to listen to their mother’s voice over another women’s voice.
Hazards to Prenatal Development
- In the United States, the best estimate is that around 45% or more pregnancies end in
miscarriage prior to the 3 week before the women has any idea she is pregnant.
- A vast array of environmental agents has the potential to cause harm during the prenatal
- Teratogen: an external agent that can cause damage or death during prenatal development
- Sensitive period: the period of time during which a developing organism is most sensitive to the
effects of external factors; prenatally, the sensitive period is when the fetus is maximally
sensitive to the harmful effects of teratogens.
- Dose-response relation: a relation in which the effect of exposure to an element increases with
the extent of exposure (prenatally, the more exposure a fetus has to a potential teratogen, the
more severe its effect is likely to be)
- The presence of multiple factors can have a cumulative impact.
- The later emergence of effects of prenatal experience is referred to as fetal programming,
because experiences during the prenatal period “program the physiological set points that will
govern physiology in adulthood”.
- The effects of teratogens can also vary according to individual differences in genetic
susceptibility (probably in both the mother and the fetus)
- Identifying teratogens is further complicated by the existence of sleeper effects, in which the
impact of a given agent may not be apparent for many years. Legal Drugs
- The two legal “drugs” that cause by far the most havoc for fetal development are cigarettes and
1. Cigarettes: the main consequence that maternal smoking has for the fetus is retarded
growth and low birth weight, risk of SIDS (sudden infant death syndrome), lower IQ, hearing