HUMAN BIOLOGY

342 Chapter 17

Birth and Beyond

The hormonal flood is triggered by an as-yet-unknown

signal that says, in effect, it’s time to be born. The placenta

responds to the fetal hormones by producing more estro-

gen. Rising estrogen in turn calls for a rush of oxytocin and

of prostaglandins (also produced by the placenta), which

jointly stimulate uterine con tractions. For about the next

2 to 18 hours, the contractions will become stronger, more

painful, and more frequent.

Labor has three stages

Labor is divided into three stages that we can think of

loosely as “before, during, and after.” In the first stage,

uterine contractions push the fetus against its mother’s

softened cervix. Initial contractions occur about every

15 to 30 minutes and are relatively mild. As the cervix

gradually dilates to a diameter of about 10 centimeters

(4 inches, or “5 fingers”), contractions become more fre-

quent and intense. Usually, the amniotic sac ruptures dur-

ing this stage, which can last 12 hours or more.

The second stage of labor, actual birth of the fetus,

typically occurs less than an hour after the cervix is fully

dilated. This stage is usually brief—under 2 hours. Strong

contractions of the uterus and abdominal muscles occur

every 2 or 3 minutes, and the mother feels an urge to push.

Her efforts and the intense contractions move the soon-

to-be newborn through the cervix and out through the

vaginal canal, usually head first (Figure 17.16). Complica-

tions can develop if the baby begins to emerge in a breech

position—that is, if a body part other than the head is exit-

ing first. In that case the doctor may use hands or forceps

to aid the delivery.

After the baby is born, the third stage of labor gets

under way. More uterine contractions force fluid, blood,

and the placenta (now called the afterbirth) from the moth-

er’s body. The umbilical cord—the lifeline to the mother—

is now severed. A lifelong reminder of this separation is

the scar we call the navel, the site where the umbilical cord

was attached.

Without the placenta to remove wastes, carbon dioxide

builds up in the baby’s blood. Together with other factors,

including handling by medical personnel, this stimulates

control centers in the brain, which respond by triggering

inhalation—the newborn’s crucial first breath.

As the infant’s lungs begin to function, the bypass ves-

sels of the fetal circulation begin to close, soon to shut

completely. The fetal heart opening, the foramen ovale,

normally closes slowly during the first year of life.

Most full-term pregnancies end in the birth of a

healthy infant. Yet babies born prematurely—especially

before about 8 months of intrauterine life—can suffer

complications because their organs have not developed

n    Birth, or parturition, takes place about 280 days from the

start of the woman’s last menstrual period.

n Link to Hormones of the hypothalamus and pituitary 15.3

maternal changes and fetal hormones set

the stage for childbirth

A pregnancy is considered “full term” about 39 weeks

after fertilization. A few weeks before this point, changes

occur that set the stage for childbirth. To begin with, col-

lagen fibers in the connective tissue of the cervix loosen.

The once-rigid cervix gradually softens so it can stretch

to allow the fetus to pass from the uterus into the birth

canal. Expectant mothers often are keenly aware of another

change—the “drop” of the fetus as it shifts to a lower posi-

tion in the uterus, usually head-down.

The birth process, or labor, begins when smooth muscle

in her uterus starts to contract. These contractions are the

indirect result of a cascade of hor-

mones from the fetus’s hypothala-

mus, pituitary, and adrenal glands.

dilating cervix

placenta

wall of uterus

umbilical cord

placenta detaching

from wall of uterus

umbilical cord

F i g u r e 17.16 During birth, the fetus is pushed out of the uterus.

The afterbirth—the placenta, fluid, and blood—is expelled shortly

afterward. (© Cengage Learning)

1 7. 9

labor The birth process.

1 Fetus positioned

for childbirth; its

head is against the

mother’s cervix,

which is dilating

(widening).

2 Muscle contrac-

tions stimulated by

oxytocin force the

fetus out through the

vagina.

3 The placenta

detaches from the

wall of the uterus

and is expelled.

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