|
Fetus:
For vertebrates that have live births rather than external eggs (these
include some fish and reptiles and most mammals), the fetus is the name given
to the developing animal between the embryo stage and birth. In humans an
implanted embryo graduates to the fetal stage after five, eight, nine, or
twelve weeks, depending on which expert you consult.
Size and location The fetus grows in size from about 2 to 3 inches long to
about 20 inches long. Just before birth the fully developed fetus ranges in
size from a low of 5½ pounds to as much as 15 pounds in extreme instances.
The typical American baby weighs from 6 to 8 pounds at birth. Below a weight
of 5½ pounds, physicians identify the birth as premature (even if full term).
All of the growth before birth takes place within the uterus. The fetus and
its mother's body cooperate to develop a special sac that is filled with
amniotic fluid. The fetus is attached to its mother by the umbilical cord,
which permits fluids, nutrients, and oxygen to pass from the mother to the
fetus and the wastes from fetal growth and development to pass back to the
mother. There is no direct exchange of blood, however. A special organ called
the placenta permits the exchange; a few blood cells sometimes do escape from
mother to fetus or vice versa.
Role The human fetus, like that of other mammals that have a placenta, is well
protected. During fetal development an individual produces functional organs
and prepares for life in the open atmosphere and a new way of obtaining
nourishment. At birth the umbilical cord is cut and the lungs are put into
action.
Conditions that affect the fetus Although the fetus is well protected by the
placenta and by its secure place within the amniotic membrane, within the
uterus, within its mother, a few viruses, bacteria, and chemicals can pass
through the barrier to interfere with development. The best known of the
viral agents that can interfere with development is German measles (rubella).
A bacterium that can cross the placenta and infect the fetus is the one that
causes syphilis. Chemicals that have induced errors in fetal development
include thalidomide, which interferes with the growth of fetal limbs. Chemicals
that permit the fetus to grow but change the pattern of growth in this way
are called teratogens. Poor maternal nutrition can retard or sometimes alter
development of the fetus.
For many errors in fetal development the exact cause is not known. In some
cases there may be genetic or chromosomal disorders and in others the problem
may be an unidentified virus.
When the fetus does not develop properly, one possible event is spontaneous
abortion or miscarriage. The fetus is expelled from the body and pregnancy is
terminated. At later stages a fetus that has developed in a normal way may
be born before the full growth term of 38 weeks. After about 25 weeks of
development, such a premature baby has a good chance of survival. Although
there may be some physical problems that persist throughout life, especially
with eye development, most premature infants are able to grow out of their
early difficulties.
|
