to Williams Obstetrics, a primary textbook in the obstet-
rical field, the milestones in fetal development can be
marked every four weeks of the fetus’s menstrual/
gestational age.
At twelve weeks from the last menstrual period,
the fetus is clearly visible by transvaginal ultrasound
and may be visualized by abdominal technique de-
pending on the quality of the equipment and the size
of the expectant mother. Fingers and toes are differ-
entiated from each other, and fingernails are present.
The external genitalia are starting to develop, but it
is difficult to determine gender at this point.
By sixteen weeks, the fetus can be seen by abdom-
inal ultrasound. The fetus now weighs approximately
110 grams (four ounces) and has well-developed
lower limbs. Intestines, stomach, and bladder should
be visible. The sex of the baby can be reliably deter-
mined at this point, with fetal cooperation, of course.
At this time, most pregnant women undergo screen-
ing tests of their blood to check for possible problems
with Down syndrome or spina bifida. An ultrasound
to assess anatomy and to look for any signs of struc-
tural defects is usually done at this age.
The end of the twentieth week represents the
midpoint of pregnancy. The average fetus weighs ap-
proximately 320 grams (eleven ounces) and is ap-
proximately sixteen centimeters (six and one-quarter
inches) long. At this point, some scalp hair may be
seen in ultrasound images, and the body is covered
with a fine, downy hair called lanugo.
The twenty-fourth week represents a major mile-
stone as hospitals with high-tech (level 3) neonatal in-
tensive care nurseries and neonatal specialists
consider fetuses at this age to be viable. Unfortunate-
ly, fetuses born at this age are extremely premature.
The skin is wrinkled, with small amounts of subcuta-
neous (below the skin) fat present; thus, they have tre-
mendous problems maintaining body temperature.
They weigh only 630 grams (twenty-two ounces) and
have poorly developed lungs, which necessitates the
use of ventilators to assist breathing.
At twenty-eight weeks, the fetus is now roughly
twenty-five centimeters (nine and three-quarter inch-
es) long and can weigh approximately 1,100 grams
(two pounds, six ounces). By this point, the fetus that
is delivered will have eyes partially open, limbs that
can move energetically, and may be able to cry. Most
fetuses born at this point will survive under the care
of a high-tech, level 3 neonatal intensive care unit.
By thirty-two weeks, the average fetus weighs
roughly 1,800 grams (three pounds, fifteen ounces),
and measures approximately 28 centimeters. The
skin is wrinkled and red, but the body begins to fill
out with more deposition of subcutaneous fat. This
represents another major milestone as the chances of
the other problems of prematurity, such as hemor-
rhages in the brain or the eye and problems with the
intestines, drop considerably. The biggest problems
facing babies born at this age involve lung develop-
ment and function.The thirty-sixth week represents another impor-
tant milestone. At this point, the baby measures ap-
proximately thirty-two centimeters (twelve and one-
half inches) and weighs approximately 2,500 grams
(five pounds, eight ounces). The body is filled out with
subcutaneous fat. Although babies born at this age are
still technically ‘‘preterm’’ (infants are considered
term at thirty-seven to forty weeks), most women who
go into labor at this age would not be stopped with
medications. Babies born at this age have an excellent
chance of survival.Finally, at forty weeks, the goal of gestation is
reached. At this age, the average baby measures thir-
ty-six centimeters (fourteen inches) and weighs ap-
proximately 3,400 grams (seven pounds, seven
ounces). The skin is smooth and pink, the body is
plump, and the lungs generally function well. It
should be noted that these stages of development rep-
resent general characteristics only. There is always a
large degree of variability in fetal development, much
of which is influenced by variable factors such as ge-
netic makeup (e.g., chromosomal defects), character-
istics of the parents (e.g., size, weight, race) and
characteristics of the pregnancy (e.g., toxemia, mater-
nal smoking, bleeding during the pregnancy). At
times, the gestation may go past forty weeks. This oc-
currence is not uncommon. In fact, very few deliveries
occur at exactly forty weeks. In cases where pregnan-
cies go ‘‘post-term,’’ the women should be followed
normally. Most obstetricians will institute measures to
induce delivery if the woman does not go into labor
on her own past forty-two weeks. At this point, there
is concern that the fetus will grow too big and an in-
creased chance of cesarean delivery. Furthermore,
the placenta has a finite ‘‘lifespan,’’ and gestations
that continue too long past the due date have a higher
risk for placental failure, which could harm the safety
of the fetus.Although the gestation itself represents a relative-
ly short period in a woman’s life, the physical and per-
sonal changes that pregnancy brings about in a
woman last throughout her lifetime. Clearly, preg-
nancy is a task that should not be met without prepa-
ration and assistance.See also: BIRTH; CONTRACEPTION; PRENATAL CARE;
PRENATAL DEVELOPMENT; REPRODUCTIVE
TECHNOLOGIESPREGNANCY 321