The process of lactation, the body’s method of producing milk, is often perceived to begin immediately following childbirth. However, the physiological mechanisms governing milk production are initiated much earlier in the reproductive cycle. Preparation for nourishing an infant starts well before the third trimester. This article clarifies the distinct stages of this physiological journey, outlining precisely when the ability to produce milk is developed.
Lactogenesis I: Milk Production Begins in Pregnancy
The initial phase of milk production, known as Lactogenesis I or secretory differentiation, begins around the middle of pregnancy. This stage typically starts between the 16th and 22nd week of gestation, marking when mammary cells become capable of synthesizing milk components. Specialized cells within the breast’s alveoli differentiate into secretory cells, preparing the glandular tissue to produce the first form of milk, known as colostrum.
Colostrum is a nutrient-dense, concentrated fluid, not the copious, high-volume mature milk seen after birth. It is produced in minimal volumes during this phase and often goes unnoticed. Because the breast is still in its preparatory stage, the initial production is characterized by low rates of synthesis. The capacity to secrete milk established during Lactogenesis I is a permanent change.
Hormonal Regulation: The Role of Progesterone and Prolactin
The commencement of milk production during pregnancy is governed by the interplay between two primary hormones: prolactin and progesterone. Prolactin, released by the pituitary gland, increases steadily throughout pregnancy and acts as the primary stimulator of milk-making cells. This hormone signals the mammary glands to begin synthesizing colostrum components, effectively triggering Lactogenesis I.
Despite the rising levels of prolactin, the body is prevented from producing large volumes of milk by high concentrations of progesterone. Progesterone, produced by the placenta, acts as a potent inhibitor or “brake” on the full secretory function of the breast tissue. This inhibitory effect ensures that milk production remains low until the pregnancy is complete.
Physical Signs: Identifying Colostrum and Prenatal Leakage
While the physiological process is underway, the physical evidence of Lactogenesis I is highly variable among pregnant individuals. Colostrum is typically thick in consistency and can range in color from clear to a deep yellow or orange hue. This color is due to its high concentration of immune factors and beta-carotene.
Some people may notice small amounts of colostrum leaking from the nipples, or observe dried, crusty residue on the areola or inside a bra. This leakage is a direct manifestation of the underlying secretory activity. Experiencing prenatal leakage is perfectly normal and confirms that the mammary glands are functioning as intended. Conversely, many people never experience noticeable leakage during pregnancy, and this absence has no bearing on the ability to produce milk after birth.
The Postpartum Shift: Transition to Full Milk Supply
The production of colostrum during pregnancy is only the first stage; the transition to a full, copious milk supply requires a dramatic hormonal shift after birth. This transition is known as Lactogenesis II, or secretory activation. The crucial signal that triggers this shift is the delivery of the placenta.
Once the placenta is expelled, the primary source of progesterone is removed, causing circulating progesterone and estrogen levels to drop rapidly. This sudden withdrawal of inhibitory hormones removes the “brake” that had been suppressing milk volume throughout the pregnancy. High levels of prolactin are then able to fully activate the milk-making cells, leading to a significant increase in production. This phase, commonly referred to as the milk “coming in,” typically begins between 30 and 72 hours postpartum, marking the shift from colostrum to higher-volume transitional milk.