Amniocentesis is a prenatal test in which fluid is extracted from the amniotic sac via the uterus of a
pregnant woman. The results of the test can indicate whether or not the infant will have a chromosomal
problem, such as a genetic disease. (Pete Saloutos/The Stock Market)should be careful about their weight gain as they have
an increased risk of producing a large for age (macro-
somic) baby, with its associated higher risk of difficult
delivery and cesarean delivery.
As each week of the pregnancy passes, the woman
will be able to gauge the progression of her pregnan-
cy only through the increasing size of her belly and
the amount of activity felt in her uterus. She may not
be fully aware of the extent of change occurring with
the fetus. Hidden within the woman is a process that
is no less fascinating than the changes the woman is
undergoing.
Fetal Development
The first trimester is the most critical period for
fetal development. It is at this point that the fertilized
egg begins to develop from a mass of disparate cells
into an organized whole that is truly the sum of its in-
dividual specialized parts. The start of the third week
after fertilization marks the beginning of the embry-
onic period. At this point, the fertilized egg begins to
differentiate its cells into the beginnings of the pla-
centa and the body of the future fetus. By the end of
the fourth week after ovulation, the embryo is roughly
four to five millimeters (0.16 to 0.2 inches) long, and
heart activity can be seen on ultrasound. By the end
of the sixth week, the embryo is roughly two centime-
ters (three-quarters of an inch) long and has a defini-
tive head separate from the body. A vast amount of
organ development occurs before many women real-
ize that they are indeed pregnant, highlighting the
importance of attention to the health of women as
they anticipate pregnancy.
The fetal period is usually considered to start by
the eighth week after fertilization. By this time, the
period most crucial to organ and structural develop-
ment has passed. Development from this period con-
sists of the growth and maturation of structures that
were formed during the embryonic period. According320 PREGNANCY