The biological process of producing milk, known as lactation or lactogenesis, is a complex, hormone-driven transformation that begins long before a baby’s birth. Changes in the breast tissue start early in pregnancy as the body responds to a surge of hormones. These hormones signal the mammary glands to develop and prepare for their primary function, ensuring nourishment is immediately available for the newborn.
The Initial Phase: Glandular Development and Colostrum Production
Functional milk production, specifically the first stage known as Lactogenesis I, typically begins midway through the pregnancy, often cited as starting between 12 and 20 weeks of gestation. During this time, the mammary glands undergo significant development, with the hormone prolactin stimulating the milk-producing cells, called lactocytes, to begin synthesis. The high levels of progesterone circulating in the pregnant body, however, act as a brake, preventing the full, copious release of milk.
The substance produced during this initial phase is colostrum, a thick, yellowish fluid. Colostrum is packed with immune factors, antibodies, and specialized proteins, making it the ideal first food for a newborn. High progesterone levels from the placenta ensure that the volume remains low, preventing the full milk “let-down” that occurs after birth.
Post-Delivery Transition: The Milk “Coming In”
The major transition to a large volume of milk production, referred to as Lactogenesis II, is triggered by the delivery of the placenta. This event causes a rapid drop in progesterone levels in the mother’s bloodstream. With the inhibitory effect of progesterone removed, high levels of prolactin fully activate the mammary glands, leading to a noticeable increase in milk volume.
This process is commonly known as the milk “coming in,” and it typically occurs between 2 to 5 days after birth. The characteristics of the milk also change during this transition from colostrum to mature milk. Colostrum gives way to transitional milk, which is higher in volume and contains more fat and lactose. By about two weeks postpartum, the milk transitions again to mature milk, which is thinner, whiter, and provides the sustained nutrition the growing infant requires.
Addressing Physical Changes and Leakage
The hormonal changes that drive milk production also cause several noticeable physical changes in the breasts throughout pregnancy. Many people experience increased breast tenderness and sensitivity, which can be one of the earliest signs of pregnancy. The breasts often increase in size as the milk ducts and glandular tissue proliferate, sometimes causing the skin to feel itchy as it stretches.
Colostrum leakage can begin as early as the second trimester, although it is not a universal experience. This leakage is a normal sign that the body is preparing for the baby’s arrival, and whether or not it occurs has no bearing on future milk supply or breastfeeding success. For managing leakage, simple solutions include placing absorbent breast pads inside a supportive bra to catch any discharge.
The areolas may also darken, and the small bumps on them, called Montgomery’s tubercles, can become more pronounced. These tubercles secrete an oily substance that helps clean and lubricate the nipple.