Lactogenesis is the physiological process by which the body initiates and maintains milk production to provide nourishment for a newborn. This process unfolds in distinct phases, driven by a precise orchestration of hormones that transform the mammary glands. The biological system adapts from pregnancy through the postpartum period, ensuring a responsive supply of milk.
Preparing the Breasts: Secretory Initiation (Lactogenesis I)
The groundwork for milk production, known as secretory initiation or Lactogenesis I, begins during pregnancy. Starting around the 16th week, the mammary glands undergo significant changes driven by pregnancy hormones. The milk ducts grow in number and size, and the breast’s internal structure remodels as alveolar cells differentiate into secretory cells capable of producing milk.
This transformation is under tight hormonal control. Estrogen stimulates the development of the ductal system, while prolactin encourages the growth of the milk-producing alveoli. Despite the breast’s readiness, high levels of progesterone from the placenta act as a brake. This hormone permits preparation but inhibits the secretion of large volumes of milk.
During this preparatory phase, the breasts produce small quantities of colostrum. This “first milk” is a dense, antibody-rich fluid that is low in volume but high in proteins and immunological components, serving as the infant’s initial source of nutrition and passive immunity. The production of colostrum signifies that the glands are ready for lactation, awaiting the hormonal signal for full production after childbirth.
Milk ‘Coming In’: Secretory Activation (Lactogenesis II)
The transition to high-volume milk production, a phase called secretory activation or Lactogenesis II, is triggered by childbirth and the delivery of the placenta. This event causes a sudden shift in the body’s hormonal landscape. With the placenta gone, progesterone levels drop rapidly, and this decline is the trigger that allows the full lactation process to begin.
This sharp decrease in progesterone “releases the brakes” on milk production, allowing the hormone prolactin to exert its full effect on the prepared mammary cells. With the progesterone inhibition lifted, prolactin signals the alveolar cells to begin synthesizing and secreting milk in large volumes. This hormonal cascade initiates what is commonly called the milk “coming in.”
This process begins around 30 to 40 hours after birth, though the sensation of breast fullness may not be felt until 50 to 73 hours postpartum. Supportive hormones, including cortisol and insulin, also support this stage by helping manage the metabolic demands of milk synthesis. The composition of the milk changes, transitioning from colostrum to a higher volume of transitional milk, which eventually becomes mature milk.
Sustaining Milk Production: Galactopoiesis
Once lactation is established, it enters a maintenance phase known as galactopoiesis, or Lactogenesis III. At this point, the control system for milk production shifts from being primarily hormonal (endocrine control) to a local, demand-based system within the breast (autocrine control). This “supply and demand” mechanism ensures that milk synthesis matches the infant’s needs.
Two hormones, prolactin and oxytocin, are important to this ongoing process. Prolactin is responsible for the continued synthesis of milk within the alveolar cells. Its release is stimulated by the infant suckling at the breast, creating a feedback loop where milk removal prompts more production for the next feeding, known as the prolactin reflex.
The second hormone, oxytocin, governs the release of milk from the breast in a process called the milk ejection reflex, or “let-down.” Nipple stimulation from suckling sends nerve signals to the brain, triggering the release of oxytocin. This hormone travels to the breast, where it causes tiny muscle cells surrounding the alveoli to contract. This contraction squeezes milk from the alveoli into the milk ducts, making it available to the infant.
The Final Phase: Weaning and Involution
The final stage of the lactation cycle is involution, the gradual process where the mammary glands cease milk production and return to a non-lactating state. This phase is initiated by the significant reduction or cessation of milk removal, as occurs during weaning. When milk is no longer regularly emptied, the system shuts down in response to the lack of stimulation.
As milk remains in the breast, a feedback mechanism inhibits further synthesis. The accumulation of milk causes pressure, and a whey protein known as the Feedback Inhibitor of Lactation (FIL) signals the secretory cells to stop producing milk. The reduced frequency of suckling also leads to a decline in prolactin levels, further diminishing the production signal.
This process is gradual, allowing the breast tissue to remodel. The milk-producing alveolar cells undergo apoptosis, a form of programmed cell death, and are cleared away by immune cells. The breast tissue then slowly returns to a state similar to how it was before pregnancy, preserving the capacity for the cycle to be re-established with a future pregnancy.