HUMAN BIOLOGY

340 Chapter 17

Development of the Fetus

In the second trimester, movements begin

When the fetus is 3 months old, it is about 4.5 inches

long. Soft, fuzzy hair called the lanugo covers its body. Its

reddish skin is wrinkled and protected from chafing by a

thick, cheesy coating called the vernix caseosa.

The second trimester of development extends from the

start of the fourth month to the end of the sixth. Figure

17.14 shows what the fetus looks like at 16 weeks. Its tiny

facial muscles now produce frowns, squints, and sucking

movements—evidence of a sucking reflex. Before the sec-

ond trimester ends, the mother can easily feel her fetus’s

arms and legs move. During the sixth month, its eyelids

and eyelashes form.

Organ systems mature during the third trimester

The third trimester extends from the seventh month until

birth. At 7 months the fetus is about 11 inches long, and

soon its eyes will open. Although the fetus is growing

larger and rapidly becoming “babylike,” it will not be able

to survive on its own until the middle of the third trimes-

ter. At 7 months few fetuses can maintain a normal body

temperature or breathe normally. However, with intensive

medical care, fetuses as young as 23 to 25 weeks have sur-

vived early delivery. A baby born before 7 months’ gesta-

tion is at high risk of respiratory distress syndrome (described

in Chapter 10) because its lungs lack surfactant and so can’t

expand adequately. The longer the baby can stay in its

mother’s uterus, the better. By the ninth month, its survival

chances are about 95 percent.

the blood and circulatory system

of a fetus have special features

The steady maturation of its organs and organ systems

readies the fetus for independent life. For the circulatory

system, however, the path toward independence requires

a detour. Several temporary bypass vessels form and will

function until birth. As Figure 17.15A shows, two umbilical

arteries inside the umbilical cord transport deoxygenated

blood and metabolic wastes from the fetus to the placenta.

There, the fetal blood gives up wastes, takes on nutrients,

and exchanges gases with the mother’s blood. Fetal hemo-

globin binds oxygen more easily than adult hemoglobin

does. This helps ensure that enough oxygen will reach

developing fetal tissues. The oxygenated blood, enriched

with nutrients, returns from the placenta to the fetus in the

umbilical vein.

Other temporary vessels divert blood past the lungs and

liver. These organs don’t develop as rapidly as some others,

because (by way of the placenta) the mother’s body can per-

form their functions. The lungs of a fetus are collapsed and

n in the second and third trimesters, organs and organ

systems gradually mature in preparation for birth.

n Links to Cardiovascular system 7.1, Red blood cells 8.1,

Gas exchange in the lungs 10.2

Figure 17.14 At 16 weeks a fetus is well formed and can move.

Movements begin as soon as nerves establish functional

connections with developing muscles. Legs kick, arms wave,

fingers grasp, the mouth puckers. These reflex actions will be

vital skills in the world outside the uterus. The drawing shows

a baby at full term—ready to be born. (© Cengage Learning)

During fetal period, length

measurement extends

from crown to heel (for

embryos, it is the longest

measurable dimension, as

from crown to rump).

Length:

Weight:

16 centimeters

(6.4 inches)

200 grams

(7 ounces)

WEEK 16

Length:

Weight:

27.5 centimeters

(11 inches)

1,300 grams

(46 ounces)

WEEK 29

Length:

Weight:

50 centimeters

(20 inches)

3,400 grams

(7.5 pounds)

WEEK 38 (full term)

placenta

placenta

Lennart Nilsson from

A Child Is Born

, © 1966, 1977 Dell Publishing Co., Inc.

1 7. 8

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