Giving Birth Requires a Complex
Series of Hormonal Changes
the events of labor and delivery.
the process of lactation.
fter fertilization has taken place, the embryo
has matured into a fetus, and the 38 weeks
of pregnancy have passed, it is time for the
baby to be born. Let’s look at how the pro-
cess of delivery of the baby is initiated and what brings
it to an end.
Labor Has Several Stages
Near the end of pregnancy, several
hormones come into play to pre-
pare for and initiate
la b o r
. High
levels of estrogens near the end of
pregnancy override the inhibitory
effects of progesterone on uterine
contractions. Estrogens stimulate
the placenta to secrete
p r o s ta -
g la n d in s
, which induce secretion
of enzymes that digest collagen
fibers in the cervix, causing it to
soften. Estrogens also induce re-
ceptors for oxytocin in uterine muscle fibers. In addition,
release of the hormone relaxin increases the flexibility of
the pubic symphysis and helps dilate the cervix.
During labor, uterine contractions force the baby’s
head into the cervix, which stretches. The stretching in-
duces more uterine contractions through a positive feed-
back mechanism involving secretion of oxytocin by the
hypothalamus (
F ig u r e 1 6 . 1 7 a
Uterine contractions occur in waves that start at the
top of the uterus and move downward, eventually expel-
ling the fetus. True labor begins when uterine contrac-
tions occur at regular intervals, usually causing pain. As
the interval between contractions shortens, the contrac-
tions intensify. Another symptom of true labor in some
women is localization of pain in the back that is inten-
sified by walking. The reliable indicator of true labor
is dilation of the cervix and the "show," a discharge of
blood-containing mucus that appears in the cervical ca-
nal during labor. In
fa ls e labor,
pain is felt in the abdomen
at irregular intervals, but it does not intensify and walk-
ing does not alter it significantly. There is no "show" and
no cervical dilation.
Labor occurs in three stages:
S ta g e o f d ila tio n .
The cervix dilates to provide room for
the baby’s head and body to pass. This stage typically
lasts 6-12 hours, features regular contractions of the
uterus, usually a rupturing of the amniotic sac, and
complete dilation (to 10 cm or about 4 inches) of the
cervix. If the amniotic sac does not rupture spontane-
ously, it is ruptured intentionally.
2 .
S ta g e o f expulsion.
The baby is expelled during this stage,
which can take anywhere from 10 minutes to several
hours from complete cervical dilation.
3 .
P la ce n ta l stage.
The placenta or “afterbirth” is expelled
by powerful uterine contractions during this stage.
These contractions also constrict blood vessels that
were torn during delivery, thereby reducing the like-
lihood of hemorrhage. (
F ig u r e 1 6 . 1 7 b
). This stage
may take 5 to 30 minutes or more after delivery.
The positive feedback mechanism
ends at childbirth, or
p a r tu r itio n
As a rule, labor lasts longer
with first babies, typically about
14 hours. For women who have
previously given
b i r t h
, labor may
last an average of 8 hours—although the time varies enor-
mously among births.
Usually, the fetus is positioned with its head down
toward the vagina. A heads-up or sideways orientation is
called a
b r e e c h
position. If the fetus is in the breech posi-
tion, the obstetrician may try to turn it manually prior to
birth or may elect to deliver the baby surgically using an
operation called a cesarean section (or C-section).
A premature baby is one who weighs less than 2500 g
(5.5 lb) at birth. Causes include poor prenatal care, drug
abuse, history of a previous premature delivery, and moth-
er’s age (below 16 or over 35). The body of a premature in-
fant is not yet ready to sustain some critical functions, and
most likely needs medical intervention to survive. The ma-
jor problem after delivery of an infant under 36 weeks of
gestation is respiratory distress syndrome (RDS), caused
by insufficient surfactant. RDS can be eased by use of arti-
ficial surfactant and a ventilator that delivers oxygen until
the infant's lungs can operate on their own.
la b o r
The process of
giving birth in which a
fetus is expelled from
the uterus through the
p r o s ta g la n d in s
lipids that are released
in small quantities and
act as a local hormone.
p a r tu r itio n
too-RISH-un) The act of
giving birth to young;
also called
c h ild b ir th
d e liv e ry .
498 CHAPTER 16
The Reproductive Systems
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