The process of producing milk, known as lactogenesis, is a complex biological sequence that begins much earlier than the moment a baby is born. Preparing the body to feed a newborn is a physiological project spanning several months of pregnancy. This timeline is governed by a precise interplay of hormones that initiates milk production and keeps it in a low-volume holding pattern until delivery. Understanding this timeline clarifies when milk production starts and the purpose of the first milk the body creates.
The Hormonal Triggers of Lactogenesis I
The initial stage of milk production, known as Lactogenesis I, starts surprisingly early, between 16 and 22 weeks of gestation. At this time, the milk-producing cells within the breast differentiate and become capable of secretion. This change is driven by a surge in prolactin, which stimulates the mammary glands to begin synthesizing the components of milk.
However, the body does not produce large volumes of milk before birth because of the high levels of progesterone. This hormone, produced by the placenta, acts as a temporary “brake” on the process, preventing the full activation of the mammary glands. Progesterone competitively inhibits the effects of prolactin, ensuring that production remains at a low, pre-delivery level. This hormonal balance allows the breasts to become ready for lactation.
Colostrum: The Body’s First Production
The substance produced during Lactogenesis I is colostrum, often described as the infant’s first immunization. This thick, sticky fluid is typically yellowish or clear and is produced in small quantities, perfectly suited for a newborn’s tiny stomach. Colostrum is fundamentally different from mature milk, serving a specialized role in the initial hours and days of life.
Its composition is highly concentrated, containing a powerful blend of immune factors, proteins, and growth factors. Colostrum is particularly rich in secretory Immunoglobulin A (IgA) antibodies, which coat the baby’s immature intestinal lining, providing immediate protection against bacteria and viruses. This protective layer acts as the first line of defense for the newborn’s immune system.
The growth factors present in colostrum also play a significant role in priming the infant’s digestive system. They help seal the gut lining, which is initially quite permeable, preventing foreign proteins and pathogens from entering the bloodstream. This unique composition makes colostrum a nutrient-dense fluid that supports the baby’s transition to life outside the womb.
The Postpartum Shift to Mature Milk
The transition from small-volume colostrum to large-volume mature milk marks the start of Lactogenesis II, a major biological shift that occurs a few days after childbirth. This change is triggered by the delivery of the placenta, which signals the end of the pregnancy’s hormonal dominance. The expulsion of the placenta causes a rapid and dramatic drop in the circulating levels of progesterone and estrogen.
With the progesterone “brake” removed, the high levels of prolactin that have been accumulating can finally exert their full effect on the mammary tissue. This hormonal green light results in the onset of copious milk secretion, often referred to as the milk “coming in.” This usually happens between 50 and 73 hours postpartum, or roughly two to three days after delivery.
The physical change is noticeable, as the breasts often feel significantly fuller, firmer, and sometimes tender, which is known as engorgement. The milk itself changes in appearance, gradually becoming whiter, thinner, and increasing dramatically in volume. Mature milk is higher in fat and lactose content compared to colostrum, providing the energy and hydration needed for the baby’s rapid growth.
Normal Leakage and When to Consult a Healthcare Provider
It is normal for some individuals to notice small amounts of colostrum leaking from their breasts during the second or third trimester. This is a common physical manifestation of Lactogenesis I and the body’s preparation for feeding. Leaking is highly variable, and some people may never notice it before delivery, while others may require the use of breast pads to manage the moisture.
If leakage is occurring, it is generally advised to let it happen naturally and avoid excessive manual expression or nipple stimulation unless instructed by a healthcare provider. While pre-delivery leakage is normal, there are certain signs that warrant immediate consultation with a doctor or midwife. These include any sudden discharge that appears bloody, foul-smelling, or is accompanied by pain, redness, or a fever. Such symptoms could suggest an underlying issue.