Physiology of Lactation
The two primary hormones that are needed for lactation are prolactin and oxytocin. Prolactin stimulates milk biosynthesis within the alveolar cells of the breast and oxytocin stimulates contraction of the myoepithelial cells that surround the alveoli, causing the milk to be ejected into the ducts leading to the nipple. Breast growth is stimulated by increasing prolactin secretion throughout pregnancy. Lactation is inhibited during pregnancy by progesterone produced by the placenta. Progesterone interferes with prolactin binding to the receptors on the alveolar cells within the breast, thereby directly suppressing milk production.[5,6] The hormonal trigger for the initiation of lactation after birth is primarily the rapid decline in the level of placental progesterone. Pharmacologic levels of estrogen (estrogen from exogenous sources vs. endogenous sources) also block prolactin activity.[5,6]
Milk secretion begins 3 to 4 days postpartum, after the steroid hormones have been cleared from maternal circulation. Suckling stimulates an increase in prolactin levels, which is important for the initiation of milk production as well as the maintenance of milk production once lactation has been established. The amount of milk produced is correlated with the amount that is removed with suckling. The optimal quantity and quality of breast milk are also dependent on other factors, such as the availability of thyroid hormone, insulin and insulin-like growth factors, cortisol, and the intake of nutrients and fluids.[5,6]
J Midwifery Womens Health. 2007;52(6):614-620. © 2007 Elsevier Science, Inc.
Cite this: Contraception and Lactation - Medscape - Nov 01, 2007.