Pregnancy and Lactation
The hormones of pregnancy, including estrogen, progesterone, prolactin, and others, cause complex changes to occur in the breast. The various hormones each play a specific part in preparing the body for breastfeeding. However, the change that the majority of women notice first can be summed up in one word: enlargement. During the first trimester of pregnancy, the ducts and alveoli in the breast multiply rapidly. The breasts may be tender, and their size increases in preparation for breastfeeding.
Lactogenesis is the term denoting the origin, or the beginning, of lactation, and it occurs in three stages. Lactogenesis I starts at about 12 weeks before delivery, as the mammary glands begin to secrete colostrum. Breast size increases further as the alveoli become filled with colostrum, but the presence of high levels of the hormone progesterone in the mother's blood inhibits the full production of milk until after birth.