Lactation

Lactation is the process of milk secretion from the mammary glands, as well as the overall period a parent produces milk to feed their young. This ability is found in all sexually mature female mammals, and may have existed before mammals evolved. The act of feeding milk is called nursing in all female creatures, and in humans it is specifically referred to as breastfeeding. Interestingly, newborn infants sometimes produce a small amount of milk from their own breast tissue, a phenomenon commonly known as witch's milk.

In most species, lactation indicates that a female has been pregnant at some point, but in humans and goats, it can occur without any prior pregnancy. Nearly all mammal species have teats, except for monotremes (egg-laying mammals), which release milk through ducts on their abdomen. Only a few species, such as certain bats, have normal male milk production.

The maintenance of ongoing milk production is called galactopoiesis, which requires the hormone prolactin. Oxytocin is essential for the milk let-down reflex, triggered by suckling. Galactorrhea is milk production unrelated to nursing, and it can happen in both males and females of many mammal species due to hormonal imbalances like hyperprolactinaemia.

The primary purpose of lactation is to provide nutrition and immune protection to offspring after birth. This allows the mother-young pair to survive even when food is scarce or too difficult for the young to obtain, expanding the range of environments the species can tolerate. The significant energy and resource investment in milk is justified by the improved survival of offspring. Lactation also brings a period of infertility—known as lactational amenorrhea in humans—which helps ensure adequate spacing between births, further increasing offspring survival odds.

In humans, from around the eighteenth week of pregnancy, hormones stimulate the growth of the milk duct system. Progesterone influences the growth of alveoli and lobes, and high levels of this hormone inhibit lactation before birth; after delivery, progesterone levels drop, triggering copious milk production. Estrogen stimulates the milk duct system to grow and differentiate, and high levels also inhibit lactation; estrogen levels fall at delivery and stay low during early breastfeeding, so estrogen-based birth control is often avoided by breastfeeding mothers to prevent a drop in milk supply. Prolactin promotes growth and differentiation of alveoli and ductal structures, and during pregnancy and breastfeeding, it increases insulin resistance, raises growth factor levels (IGF-1), and alters lipid metabolism. During lactation, prolactin maintains tight junctions in the ductal epithelium and regulates milk production through osmotic balance. Human placental lactogen (HPL), released by the placenta from the second month of pregnancy, is closely linked to prolactin and helps growth of the breast, nipple, and areola before birth. Follicle stimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin (hCG) influence estrogen, progesterone, prolactin, and growth hormone production. Growth hormone (GH) is structurally similar to prolactin and independently contributes to galactopoiesis. Adrenocorticotropic hormone (ACTH) and glucocorticoids like cortisol play a role in inducing lactation in several species, including humans, and help regulate tight junctions. Thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH) are important galactopoietic hormones that increase naturally during pregnancy. Oxytocin contracts the smooth muscle of the uterus during and after birth, and during orgasm; after birth, it contracts the band-like cells around the alveoli to squeeze milk into the duct system, making it necessary for the milk ejection reflex (let-down) in response to suckling. Lactation can also be induced without pregnancy through a combination of birth control pills, galactagogues, and milk expression using a breast pump.

During the latter part of pregnancy, the breasts enter the secretory differentiation stage, producing colostrum—a thick, sometimes yellowish fluid. High progesterone levels at this stage inhibit most milk production, and leaking colostrum before birth is not a medical concern or an indicator of future milk production.

At birth, prolactin levels remain high, while the delivery of the placenta causes a sudden drop in progesterone, estrogen, and HPL. This abrupt withdrawal of progesterone, in the presence of high prolactin, triggers the copious milk production of secretory activation. When the breast is stimulated, prolactin levels in the blood rise, peak around 45 minutes later, and return to pre-breastfeeding levels after about three hours. Prolactin release triggers milk production in the alveoli, and prolactin also transfers into the breast milk. Some research suggests prolactin in milk is higher when milk production is greater and lower when breasts are fuller, with the highest levels typically occurring between 2 a.m. and 6 a.m. Other hormones—such as insulin, thyroxine, and cortisol—are also involved, though their exact roles are not yet well understood.