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Milton Kotelchuck
PRENATAL DEVELOPMENT
The end result of a successful pregnancy is that mira-
cle called a child, which begins as a simple zygote and
becomes a fertilized ovum during the first of three
stages, or trimesters, of prenatal development. Dur-
ing the nine months, or approximately 266 days, of
prenatal development, the zygote divides into billions
of cells, which eventually become differentiated from
one another while new systems and parts become in-
tegrated.
Ovum or Germinal Stage
Almost right after conception, cell division be-
gins. While the zygote is splitting and new cells are
created, it moves through the mother’s fallopian tube
toward the uterus, the place it will call home and
where it will receive nourishment for the rest of its
prenatal days. As with any other ‘‘egged’’ living thing,
the yolk of the ovum provides all necessary nourish-
ment.
By the time the cell cluster arrives at the uterus
on the first stop of this journey, the process known as
differentiation is just beginning. Here cells separate
into groups according to their future roles. At this
point the blastocyst (fertilized ovum) is a hollow ball
of cells. Part of these cells will begin to form four
membranes that help protect the growing organism.
These membranes will eventually become the yolk
sac, the allantois (which later becomes part of the cir-
culatory system), the amnion (which soon forms the
amniotic sac, the fetus’s bubble-like home), and the
chorion, which later becomes the placenta.
The blastocyst literally ‘‘floats’’ for some time in
the uterus, and by the sixth day after conception it
finds its home by implanting itself in the uterine wall.
This is a critical point in gestation, because if the blas-
tocyst does not implant itself properly and at the right
time and in the right place, the cell mass will die be-
fore it can reach the embryo stage. If all goes well, the
blastocyst will be firmly embedded about two weeks
after conception.Embryo Stage
During the forty-six-day embryo stage, the em-
bryo grows to a length of more than one inch (2.5 cen-
timeters). By the end of the embryo stage many body
systems will be in operation, and the embryo will
begin to appear human-like.
The embryo takes nourishment and oxygen and
releases waste products through the umbilical cord,
which links it with the placenta. The umbilical cord
contains three blood vessels through which the em-
bryo’s blood circulates to and from the placenta.
The placenta is a disk-shaped mass of tissue six
to eight inches (15.2 to 20.3 centimeters) long and
one inch (2.5 centimeters) thick and weighs about one
pound (.45 kilogram). Implanted in the inner wall of
the uterus, it serves as a two-way filter between the
bloodstream of the mother and the embryo. The pla-
centa makes it possible for the mother to carry on life
functions such as digestion, excretion, circulation,
and respiration for the embryo. Into the placenta, by
way of two arteries in the umbilical cord, the embryo
deposits such waste material as carbon dioxide. The
mass of blood vessels on the mother’s side of the pla-
centa then absorbs the wastes into her bloodstream.
The embryo receives, through the vein in the umbili-
cal cord, fresh nutrients (oxygen, amino acids, sugar,
fats, and minerals) from the mother’s bloodstream,
and hormones, antibodies, and other necessary sub-
stances by the same route.
The placenta acts as a highly permeable mem-
brane and as a natural screen to keep out many—but
unfortunately, not all—harmful substances. Thus, the
unborn child receives only materials with molecules
that are small enough to pass through the screen. At
the same time that the embryo is taking shape, the
amniotic sac is developing into a protective chamber.
By the end of the eighth week this sac completely sur-
rounds the embryo. The watery fluid inside keeps the
embryo from being jostled by any sudden movements
of the mother or by accidents that may happen to her,
such as a fall. The amniotic sac also keeps the embryo
at a constant temperature.
During the embryonic period, three layers of cells
are differentiated. The outer layer, or ectoderm, de-328 PRENATAL DEVELOPMENT