The breasts become fully developed under the influence of estrogen,
progesterone, and prolactin during pregnancy. Prolactin causes the production
of milk, and oxytocin release (via the suckling reflex) causes the contraction of
smooth-muscle cells in the ducts to eject the milk from the nipple.
The first secretion of the mammary gland after delivery is colostrum. It
contains more protein and less fat than subsequent milk, and contains IgA
antibodies which impart some passive immunity to the infant. Most often it
takes one to three days after delivery for milk production to reach appreciable
The expulsion of the placenta at delivery initiates milk production and causes
the drop in circulating estrogens and progesterone. Estrogen antagonizes the
positive effect of prolactin on milk production.
The physical stimulation of suckling causes the release of oxytocin and
stimulates prolactin secretion, causing more milk production.