Though estrogen and progesterone are essential for the physical development of the breast, a unique effect of both these hormones is to inhibit milk secretion. On the other hand, prolactin does exactly the opposite—it promotes milk secretion. Prolactin is secreted by the mother’s pituitary gland with maternal blood concentration levels rising from the 5th week of pregnancy until birth. By this time prolactin’s concentration level is about 10 times the normal non-pregnant level. Furthermore, the placenta secretes large quantities of human chorionic somatomammotropin in support of prolactin’s effect. Still, the inhibitory effects of estrogen and progesterone allow only a few mililiters (mL) of fluid to be secreted each day until after the birth of the baby. With the birth of the baby both estrogen and progesterone levels decrease since secretion of these hormones by the placenta is lost. Thus, prolactin is allowed to assume its natural milk-promoting role and large amounts of human milk are produced. At the same time the mother must secrete adequate amounts of her other hormones, especially human growth hormone, cortisol and parathyroid hormone, for adequate milk production. These 3 hormones, in particular, are crucial to assure supply of amino acids, fatty acids, glucose and calcium that are required for milk formation.
During the 1st few weeks following delivery the mother’s prolactin levels return to her non-pregnant level. However a combined neurogenic & hormonal response is activated each time the mother nurses or pumps her breasts. Milk is continuously secreted into the alveoli but milk does not flow easily from the alveoli into the ductal system. This prevents a continual leak from the breasts. Instead, the milk must be ejected or “let-down” from the alveoli into the ducts before the baby can obtain it. Each time the baby nurses signals from the mother’s nipples to her hypothalamus cause about a 10-fold surge in prolactin and oxytocin secretion lasting about 1 hour. When the baby nurses at the breast, sensory impulses are transmitted from the nipples to the spinal cord and then to the hypothalamus causing the prolactin-oxytocin surge. The oxytocin is carried in the blood to the breasts where it causes the myoepithelial cells around the walls of the alveoli to contract expressing milk from the alveoli into the ducts. This surge stimulates the breasts to produce milk for the next feeding. This whole process generally takes about 30 seconds to 1 minute after the baby begins nursing. This process is known as the milk ejection or milk let-down reflex.
Some women but not all may feel the let-down reflex. The most common reported sensation is a tingling or fullness in the breasts. Other indications of the let-down reflex include uterine cramping, milk dripping from the breasts, thirst, or sleepiness. Usually after the first week or two, mothers will begin to feel or recognize their let-down reflex; some may never feel it. Assure the mother this is not uncommon or abnormal.
If for some reason the prolactin-oxytocin surge is absent or inhibited due to injury or damage to the hypothalamus or pituitary gland, or the mother doesn’t nurse, the breasts will loose their ability to produce milk within days. Thus it is essential for the breastfeeding mother to regularly nurse or pump her breasts. Nursing at one breast causes milk flow not only in that breast but also in the other breast. Also, handling or stroking of the baby by the mother or hearing the baby cry often gives enough of a signal to the mother’s hypothalamus to stimulate her let-down reflex.