and the fetus even sleeps and wakes and keeps almost
regular hours. In month six the eyelids can open, and
the fetus can open and close its eyes. It may now weigh
as much as twenty-four ounces (680 grams). During
month seven the eyes can distinguish light from dark,
and the brain has more control over body systems
than before. The most important aspect of this partic-
ular time in development is that an infant born pre-
maturely now has a fair chance for survival. In the last
two months the fetus gains about eight ounces (227
grams) per week and gets ready for birth.
Prenatal Environmental Influences
With increasingly sophisticated technology, the
fetus has been studied and is considered to be an ac-
tive agent in its own development. Many scientists be-
lieve that anything that affects the environment of the
fetus can have an effect upon development beginning
at conception and not at birth.
Environment does indeed begin to influence the
individual as soon as he or she is conceived. As the zy-
gote undergoes mitosis (cell division), the new cells
themselves become part of the mother’s environ-
ment, and through their particular physical and
chemical influence they guide and control the devel-
opment of further new cells. Different genes are acti-
vated or suppressed in each cell, so that while one
group of cells is developing into brain tissue, another
is giving rise to the heart, another to the lungs, and
another to the skeletal system. Meanwhile, the lump
of cells is surrounded by the larger environment of
the mother’s uterus, and this environment is sur-
rounded by the mother and the world in which she
lives.
The great majority of women have uncomplicat-
ed pregnancies and give birth to healthy babies, and
for many years it was believed that the baby in the
uterus was completely insulated from outside influ-
ences. Scientists now know that this is not entirely
true. Environmental influences ranging from radio-
activity and stress in the outside world to drugs, chem-
icals, hormones, and viruses in the mother’s
bloodstream can affect prenatal development.
Such threats affect the health of the fetus in sever-
al ways. This is because the body organs and parts de-
velop at different speeds and go through definite
phases. First they go through a phase of rapid multi-
plication in the number of cells. Then there is an in-
crease in both the number of cells and in cell size. In
the third and final phase of development, cell size
continues to increase rapidly, but cell division slows
down.
When a body part or organ system is growing
most rapidly both in cell number and size, this is
known as a critical period. If an environmental factor,
such as a chemical or virus, interferes with growth
during the critical period, development of that organ
system will be permanently affected.
The effect of environmental influences varies,
therefore, in accordance with the stage of prenatal de-
velopment in which the environmental factor is en-
countered, as well as the intensity of the threat, as
shown in Table 1.
During the first three months of pregnancy, tis-
sues and important body systems develop in the em-
bryo. Adverse influences during this period will affect
the basic structure and form of the body and may
have particularly serious effects on the nervous sys-
tem. Physical development can be arrested and irrep-
arable malformation may occur. For example, many
women who took the once-available drug Thalido-
mide during the first three months of pregnancy gave
birth to children with serious defects. When taken to-
ward the end of pregnancy, however, the drug did not
seem to have any negative effects.Teratogens
The scientific study of congenital abnormalities
caused by prenatal environmental influences is
known as teratology (from the Greek word teras,
meaning ‘‘marvel’’ or ‘‘monster’’), and the environ-
mental agents that produce abnormalities in the de-
veloping fetus are called teratogens.
Drugs
Chemicals (over-the-counter and prescribed
pharmaceuticals as well as illegal substances) can
cause a wide range of congenital abnormalities that
account for about 10 percent of birth defects. The se-
verity of the abnormality depends on the amount of
the chemical the mother is exposed to, the develop-
mental stage of the fetus, and the period of time over
which the mother’s exposure to the chemical takes
place.
In terms of narcotics, women who are addicted to
heroin, morphine, or methadone give birth to addict-
ed babies. Soon after birth, the babies show symptoms
of withdrawal, including tremors, convulsions, diffi-
culty breathing, and intestinal disturbances.Smoking and Nicotine
Cigarette smoking has already been shown to
have dire consequences for the smoker, and it can be
hazardous for the fetus and the newborn child. The
results of studies of thousands of pregnancies in the
United States and elsewhere, encompassing various
ethnic, racial, and cultural groupings, indicate that
the fetus and newborn are significantly affected by
cigarette smoking during pregnancy.
Maternal smoking increases the risk of spontane-
ous abortions, bleeding during pregnancy, prema-330 PRENATAL DEVELOPMENT