The glandular tissue of the female breast develops during puberty due to the action of estrogen. It is only during pregnancy that this entire development process is completed so that the glandular tissue can produce breast milk. The full breast milk production capacity, however, is only possible after birth.
Breast Milk Production During Pregnancy
The hormonal changes during pregnancy matures the glandular tissue of the breast. The number of alveoli (acini) increases as well as the fat quantity within the breast. Later in pregnancy, the matured glandular tissues are sufficiently developed to start producing breast milk.
Estrogen, progesterone and prolactin are the main hormones that regulate this developmental process along with other hormones like growth hormone, glucorticoids and even insulin. While elevated levels of estrogen and progesterone promote the maturation of the glandular tissue, prolactin is required to actually start the breast milk production process.In fact estrogen and progesterone inhibit breast milk production itself and only after birth, when the levels of estrogen and progesterone drop, can prolactin have its full effect on milk production. Therefore the breast milk production even in late pregnancy is very small.
Prolactin is secreted from the anterior pituitary and its milk producing effects are probably enhanced by human chorionic somatomammotropin from the placenta. However, it is also the placenta itself that secretes the milk-inhibiting estrogen and progesterone.
Breast Milk Production After Birth
After the birth of the baby, the levels of estrogen and progesterone drop. Colostrum, which is similar in composition to breast milk except that it has very low quantities of fat, is secreted a few days before and after birth. The quantity of colostrum available is small, almost 1% of that of the full breast milk capacity. Within a 10 days after birth, the breasts will start producing milk to nourish the infant (lactation).
Although the inhibiting action of estrogen and progesterone is removed after birth, prolactin which is essential for breast milk production also decreases in this time. However, the prolactin levels can be manipulated by nursing. When the baby suckles, nerves in the nipple relay signals to the hypothalamus to secrete thyrotropin-releasing hormone (TRH) and decrease the secretion of prolactin-inhibiting factor (PIF). TRH will stimulate the anterior pituitary to secrete prolaction, while PIF inhibits prolactin secretion. The anterior pituitary actually produces prolactin all the time but this is inhibited by the action of PIF. So when weaning, it is not the stimulation of TRH secretion that is the more significant deciding factor in prolactin secretion but rather the extent by which PIF is inhibited thereby allowing prolactin secretion to continue unabated.
During weaning, the prolactin secretion surges for about an hour. The surge is almost 10 to 20 times more than normal. However, in women who do not breastfeed, the prolactin secretion remains low and will return to the same levels as prior to pregnancy. Within a week of not breastfeeding, the glandular tissue can lose its ability to produce breast milk. This discontinues almost entirely by the seventh to ninth month after birth. It is however, not uncommon for women to lactate for years, even if there is just the odd suckling or stimulation.