High blood levels of estrogens and progesterone during pregnancy stimulate the development of the mammary glands and enlargement of the breasts in preparation for milk secretion, or lactation (lak-ta’-shun). Although the mammary glands are capable of secreting milk, the high levels of estrogens and progesterone inhibit the hypothalamus so that milk secretion does not occur during pregnancy.
After birth, the blood levels of estrogens and progesterone drop dramatically, removing the inhibitory effect and enabling the hypothalamus to secrete prolactinreleasing hormone (PRH). PRH activates the anterior lobe of the pituitary to secrete prolactin, which stimulates milk production.
Prolactin stimulates the mammary glands to secrete milk, but its effects are not evident for two to three days. In the meantime, the mammary glands produce colostrum (ko-los ‘-trum), which differs from true milk by containing higher concentrations of protein and essentially no fat. The high protein content provides an added boost of essential nutrients for protein synthesis, which is needed for the continued development and growth of the infant.
The continued secretion of both prolactin and milk are maintained by mechanical stimulation of the nipples by the suckling infant. Suckling triggers the formation of nerve impulses that stimulate the hypothalamus to secrete PRH, which, in turn, causes continued prolactin secretion by the anterior lobe of the pituitary. Thus, lactation is maintained by a positive-feedback mechanism.
Milk does not simply flow from the breasts. Instead, it is ejected after about 30 seconds of suckling by the infant. Stimulation of the nipple by suckling sends nerve impulses to the hypothalamus, which triggers the release of oxytocin by the posterior lobe of the pituitary. Oxytocin stimulates contraction of specialized epithelial cells surrounding the ducts and alveolar glands within the mammary glands, resulting in milk ejection or “letdown.”
Milk production may continue for as long as the nipple is suckled, but the volume of milk produced gradually declines. If suckling is stopped, milk accumulates, the secretion of prolactin is inhibited, and milk production ceases in about a week.
Breast-feeding seems to provide advantages for the infant, including
(1) Better nutrition because nutrients are easier to absorb
(2) Rapid bonding due to prolonged contact with the mother
(3) Antibodies that prevent digestive inflammation and provide defense against pathogens
(4) Enhanced cognitive development.