During pregnancy, a number of changes occur over time which prepare the breast for lactation.

Early in the first trimester, the breasts and nipples become tender. The tenderness persists until the end of the 1st trimester, at which time the tenderness disappears.

By the end of the first trimester, enlargement of the breast and nipple is noticeable.

By the third trimester, the breast and nipple have experienced further enlargement and the Montgomery’s glands around the periphery of the areola become more pronounced.

The nipples gradually darken, becoming dark brown or black by full term.

While these visible changes are occurring, endocrinologic changes are preparing the breasts for lactation, requiring estrogen, progesterone, insulin, cortisol, prolactin and human placental lactogen. Ultimately, the breasts are prepared for the alveoli to secrete milk.