down beneath the skin. The testes of males descend from
regions near the developing kidneys, through the inguinal
canal, and into the scrotum (see chapter 22, pp. 833–835).
The digestive and respiratory systems mature last, which is
why premature infants may have difF culty digesting milk
Premature infants’ survival chances increase directly with age and
weight, and parallel increasing maturity of the lungs. The ability of the
alveoli to exchange gases and the presence of surfactant to reduce
alveolar surface tension are important. A baby born at 25 weeks has
a 50% chance of survival; at 24 weeks, 39% survival; and at 23 weeks,
17% survival. One of the smallest and earliest premature babies is
Amillia Taylor, born at 21 weeks, 6 days and weighing slightly under
10 ounces (about 280 grams).
Approximately 266 days after a single sperm burrowed its
way into a secondary oocyte, a baby is ready to be born,
It is about 50 centimeters long and weighs 2.7 to 3.6
kilograms. The skin has lost its downy hair but is still coated
with sebum and dead epidermal cells. The scalp is usually
covered with hair; the F ngers and toes have well-developed
In the fourth month, the body grows rapidly and reaches
a length of up to 20 centimeters and weighs about 170 grams.
The lower limbs considerably lengthen, and the skeleton
continues to ossify. The fetus has hair, nipples, and nails,
and may even scratch itself.
In the fifth month, growth slows. The lower limbs
achieve their F
nal relative proportions. Skeletal muscles con-
tract, and the pregnant woman may feel fetal movements for
the F rst time. Some hair grows on the fetal head, and F ne,
downy hair called lanugo covers the skin. A cheesy mixture
of sebum from the sebaceous glands and dead epidermal
cells (vernix caseosa) also coats the skin. The fetus, weigh-
ing about 450 grams and about 30 centimeters long, curls
into the fetal position.
During the sixth month, the fetus gains substantial
weight. Eyebrows and eyelashes appear. The skin is wrin-
kled and translucent. Blood vessels in the skin cause a red-
In the seventh month, the skin becomes smoother as fat
is deposited in the subcutaneous tissues. The eyelids, which
fused during the third month, reopen. At the end of this
month, the fetus is about 40 centimeters long.
In the F nal trimester, fetal brain cells rapidly form net-
works, as organs specialize and grow. A layer of fat is laid
During development, body proportions change considerably.