From the sixth month of pregnancy, the gland cells in the breast begin to secrete. The secretion in these early stages is known as colostrum. This secretion contains more protein than fat and is made up of protein im-munoglobulins (antibodies) that help fight infection.
Human breast milk is largely made up of water (88.5 g/dL), lactose (6.8 g/dL), fat (3.8 g/dL), various ions such as sodium, potassium, chloride, calcium, magne-sium, phosphorus, iron (very little), and vitamins. It also contains enzymes with antibiotic properties. It is shown that infants who are breast-fed have fewer in-fections than those who have been given formula from a bottle. This is because breast milk also contains im-munoglobulins, lysosomes, neutrophils, and macro-phages. A baby of normal size drinks about 850 mL, and the breasts secrete milk according to demand. Lac-tation may continue for 2 to 3 years after parturition if sucking occurs at regular and frequent intervals.
The reflex by which milk is secreted when an in-fant suckles the nipples is known as the milk let-down reflex. Here, the sensory nerves around thenipple are stimulated and, as a result of communica-tion with the pituitary gland, result in oxytocin secre-tion. The bloodstream carries oxytocin to the breasts where it causes the myoepithelial cells (smooth mus-cle cells located around the mammary glands and ducts) to contract and expel the milk.
Breast-feeding reflexively inhibits the secretion of GnRH from the hypothalamus and gonadotropins from the pituitary and, thereby, ovulation. Breast-feeding can be considered a natural form of contra-ception. However, it is not an effective form of con-traception in those who breast-feed infrequently, with wide intervals between feedings.
During lactation, the mother’s diet has to be ade-quate because she uses large amounts of fat and pro-tein to produce milk. In addition, if her calcium in-take is insufficient, her parathyroid glands stimulate the absorption of calcium and phosphates from her bones, weakening them.