17.18: Lactation

Lactation is a process by which the mammary glands of the postpartum female breast synthesize and release milk in response to infant suckling. Breast milk, considered the ideal source of nutrition and passive immunity for the infant, offers numerous benefits. It stimulates mild uterine contractions that assist in returning the uterus to its pre-pregnancy size, a process known as involution. Additionally, lactation significantly increases the mother's metabolism, aiding in using fat reserves accumulated during pregnancy.

The anterior pituitary hormone prolactin is central to the initiation and maintenance of breast milk supply. Prolactin plays a crucial role in ensuring a continuous milk supply and in mobilizing maternal micronutrients for breast milk production. After giving birth, the baseline prolactin level declines substantially. However, it experiences a brief surge, lasting about an hour, during each feeding session to stimulate milk production for the subsequent feeding. With each prolactin surge, there are slight increases in estrogen and progesterone levels.

When the infant suckles, sensory nerve fibers in the areola trigger a neuroendocrine reflex, this reflex leads to the secretion of milk from specialized cells called lactocytes into small sacs known as alveoli. Simultaneously, the posterior pituitary releases oxytocin, which prompts myoepithelial cells to contract and expel milk from the alveoli. This milk travels through lactiferous ducts, accumulates in lactiferous sinuses, and eventually exits through nipple pores. Astonishingly, the entire process occurs in under one minute, from the infant's initial suckling (the latent period) to the milk's release.

The synthesis of milk under the influence of prolactin evolves over time. Frequent milk removal through breastfeeding or pumping will help maintain elevated prolactin levels for several months. However, even with ongoing breastfeeding, the baseline prolactin level gradually decreases to its pre-pregnancy state. Beyond prolactin and oxytocin, other hormones, such as growth hormone, cortisol, parathyroid hormone, and insulin, also contribute to lactation. These hormones facilitate the transport of maternal amino acids, fatty acids, glucose, and calcium to breast milk, further enhancing its nutritional value for the infant.²⁹

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