When a woman becomes pregnant, levels of progesterone, prolactin and placental lactogen rise. The internal mammary tissue undergoes a remarkable expansion. This begins stage one lactogenesis.
During the latter part of pregnancy, the breasts are making colostrum, but high levels of progesterone keep the breast from making milk and keep the amount low. Around the time of birth, the circulating progesterone begins to drop.
At birth, the delivery of the placenta results in a quick drop in progesterone levels and stage two lactogenesis or milk secretion begins.
This abrupt withdrawal of progesterone in the presence of high prolactin levels cues lactogenesis II (copious milk production).
Although lactogenesis II begins approximately 30-40 hours after birth, mothers do not typically start feeling increased breast fullness (the sensation of milk coming in) until 50-73 hours (2-3 days) after birth.
These two stages of lactation are hormonally driven – they occur whether a mother is breastfeeding her baby or not.