HUMAN BIOLOGY

Development anD aging 343

Recent years have seen a significant increase in

births by cesarean section, in which a full-term fetus

is surgically removed from the mother’s uterus. In

2012 in the United States, to the growing alarm of

some health authorities, nearly 33 percent of babies

were born this way instead of via vaginal birth. Use

Web resources such as http://www.childbirthconnection

.org to research this topic. What conditions may

necessitate a surgical delivery? Why are some

health care authorities concerned about overuse

of the procedure?

THiNk OuTSidE THE BOOk

What are basic features of labor and

lactation?

• The mother’s cervix dilates during the first stage of labor.
  The baby is born during the second stage. In the third stage,
  contractions of the uterus expel the placenta.


• Lactation, or milk production, begins a few days after birth. It
  is stimulated by the hormone prolactin, which is released from
  the mother’s pituitary and acts on her breast (mammary gland)
  tissues.


• Suckling triggers a reflex in which oxytocin from the pituitary
  acts to force milk into mammary ducts. The response continues
  as long as the infant suckles.

taKe-Home message

lactation The production of

milk by mammary glands in

a female’s breasts.

Human milk is a complex substance. It consists mainly

of the proteins, the sugar lactose, and fats that an infant

can easily digest. Other components in the milk include

vitamins and minerals, some digestive enzymes, and mater-

nal antibodies that will help protect the infant from infec-

tions during its early life outside the womb.

The “let-down” or flow of milk

from a nursing mother’s mammary

glands is a reflex and an example

of positive feedback (Figure 17.18).

When a newborn nurses, mechano-

receptors in the nipple send nerve impulses to the hypo-

thalamus, which in turn stimulates the mother’s pituitary

to release oxytocin, which causes the mother’s breast

tissues to contract. This forces milk into the ducts. This

response continues as long as the baby suckles. Oxytocin

also triggers contractions of uterine muscle that will help

to “shrink” the uterus back to its normal size.

to the point where they can function independently.
Then, attempts to sustain the baby’s life under conditions
that will permit the necessary additional development
may require a variety of advanced medical technologies.
Even then, the majority of extremely premature infants
don’t survive.

Lactation is milk production in a mother’s
mammary glands

During pregnancy, estrogen and progesterone stimulate
the growth of mammary glands and ducts in the mother’s
breasts (Figure 17.17). Prolactin secreted by the woman’s
anterior pituitary helps stimulate the synthesis of milk. In
the first few days after birth, her mammary glands pro-
duce colostrum, a pale fluid that is rich in proteins, anti-
bodies, minerals, and vitamin A. Then prolactin secreted
by the pituitary stimulates milk production, or lactation.

A B

nipple

adipose

tissue

milk-producing

mammary gland

milk duct

Figure 17.17 Female breasts contain milk glands. A The

anatomy of the breast of a non-lactating woman. B Breast

of a lactating woman. (© Cengage Learning)

suckling by infant

stimulates mechano-

receptors in nipple

nerve impulses to

hypothalamus

milk released into

ducts

hypothalamus

signals posterior

pituitary to release

oxytocin

oxytocin stimulates

contraction of

breast tissue

positive feedback

Figure 17.18 Positive feedback keeps milk flowing

to a suckling infant. (© Cengage Learning)

Phanie/Science Source

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