About 7% of pregnant women do not deliver by 2
weeks after their due date. Post-term pregnancies carry
an increased risk of brain damage to the fetus, and even
fetal death, due to inadequate supplies of oxygen and nu-
trients from an aging placenta. Delivery may be facilitated
by inducing labor, initiated by administration of oxytocin,
or by surgical delivery through a cesarean section.
After the delivery of the baby and placenta, the moth-
er's reproductive organs and physiology return to their pre-
pregnancy state. This process usually takes about 6 weeks.
Following delivery, the baby can feed on its mother’s milk.
Let’s take a closer look at this process.
Lactation Provides Food
for the Newborn
Milk production by mammary glands is maintained by the
hormone prolactin (PRL), which is secreted by the an-
terior pituitary. Although prolactin stimulates milk pro-
duction, the mammary glands do not produce milk during
pregnancy because of inhibition by high levels of estrogens
and progesterone. When levels of these hormones de-
crease after birth, the mammary glands can produce milk.
L a c ta tio n
occurs via a positive
feedback
mechanism
involving
the hormone oxytocin. The reflex
involves the baby’s suckling action
on the mother’s nipples, the hypo-
thalamus, the anterior pituitary, and the mammary glands
(Figure 16.18). Oxytocin causes the release of milk into
the mammary ducts. Milk formed by the glandular cells of
the breasts is stored until the baby begins active suckling.
Stimulation of touch receptors in the nipple initiate sen-
sory nerve impulses that are relayed to the hypothalamus.
In response, secretion of oxytocin from the posterior pitu-
itary increases.
During the first few days postpartum, the mammary
glands
secrete
a
nutrient-rich
fluid called
c o lo s tr u m
. Compared
to milk, colostrum has less lactose
and no fat; however, it is easily di-
gested and helps the baby survive
until true milk comes in about 4
days after birth. Colostrum has
a very high protein content. The
primary proteins are antibodies, made by the immune
system and released into the colostrum, which the baby
is able to absorb intact during the first few days of life.
These antibodies are the baby’s main source of adaptive
immunity (see Chapter 12) until its own immune system
begins to manufacture additional antibodies several weeks
or months after birth.
Frequent suckling by the baby (8 to 10 times per day)
can block ovarian cycles during the first few months af-
ter birth; however, this is not a reliable form of birth con-
trol. The effect is inconsistent, and ovulation commonly
precedes the first menstrual period after the delivery of a
baby. As a result, the mother can never be certain she is
not fertile.
The primary benefit of breastfeeding is nutritional:
Human milk is a sterile solution that contains fatty acids,
lactose, amino acids, minerals, vitamins, and water that
are ideal for the baby’s digestion, brain development, and
growth. As already noted, breast milk contains maternal
antibodies, which help protect the baby from infection.
Breast milk also contains some immune cells, including
T lymphocytes and macrophages, which attack invading
microbes directly. In fact, health care providers in both in-
dustrialized and developing countries have long observed
that breast-fed babies contract fewer infections than ba-
bies fed with formula. Breast milk is more nutritious and
beneficial to the baby than commercial formulas.
The mother also receives benefits from breastfeed-
ing her baby. Breastfeeding helps her lose weight gained
during the pregnancy much more quickly than if she does
not breastfeed. In addition, she is likely to experience less
postpartum bleeding, and her uterus returns to its pre-
pregnancy size more rapidly. There is even some evidence
that breastfeeding may reduce the risk of breast and re-
productive organ cancers.
Years before oxytocin was discovered, it was common
practice in midwifery to let a first-born twin nurse at the
mother’s breast to speed the birth of the second child.
Now we know why this practice is helpful—it stimulates
the release of oxytocin. Even after a single birth, nursing
promotes expulsion of the placenta and helps the uterus
return to its normal size. Synthetic oxytocin is often given
not only to induce labor but also to increase uterine tone
and control hemorrhage just after giving birth.
CONCEPT CHECK
1.
What
happens during the first stage of labor?
2.
How
does the baby's suckling stimulate milk
ejection?
L a c ta tio n
(lak-TA-
shun) The secretion
and ejection of milk by
the mammary glands.
c o lo s tr u m
(ko-LOS-
trum) A thin, cloudy
fluid secreted by the
mammary glands a
few days and after
delivery, before true
milk is produced.
500 CHAPTER 16
The Reproductive Systems
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