Pregnancy causes significant changes in the breast in preparation for feeding the newborn infant.  Lactation requires a complex series of events involving hormones and changes in milk-producing glands.  The development of lactation requires one final factor - a hungry infant or removal of breast milk with a pump.

When do the female milk-glands develop?

The breast tissue begins development in the female fetus at around 4 weeks of gestation.  The mammary (milk-producing) glands develop as branching tree-like structures with "buds" at the tips of the branches.  At the time of birth these buds often produce a small amount of milk in response to high hormone levels in the mother and placenta.  Drops of milk may be visible at the nipples of newborns (even in boy infants!) which is sometimes called "witch's milk."  Growth of the mammary glands dramatically increases at the time of puberty and milk production occurs during pregnancy.

What is the structure of mammary glands?

The mammary glands are tree-like structures with tiny sacs called alveoli at the tips of the branches.  These small sacs contain the milk-producing cells and milk is stored in these sacs during lactation.  When the breast is stimulated by hormones, muscles around the alveoli contract and force the milk into connecting ductules.  These ductures meet to form larger ducts that exit the breast at the nipple.  About 5 to 10 ducts have visible openings on the outside tip of the nipple.

How does the female breast change during pregnancy?

During pregnancy, the average breast size doubles to about one pound, the breast skin appears thinner, veins under the skin become more visible, and the areola and nipple darken and enlarge.

What stimulates the production of breast milk?

During the third trimester of pregnancy, the mammary glands produce a high-protein milk called "colostrum."  After delivery of the infant, the hormone progesterone drops while the level of prolactin stays elevated.  This stimulates the production of normal breast milk.  Breast milk production is stimulated by multiple hormones after the birth of the infant, mostly prolactin.  Oxytocin is released from the pituitary gland in response to nipple stimulation by a feeding infant.  Oxytocin stimulates the ejection of breast milk from the mammary glands and is important for maintaining milk production.   

Photo  - The Lactating Breast - When the baby sucks, a hormone called oxytoxin starts the milk flowing from the alveoli, through the ducts (milk canals) into the sacs (milk pools) behind the areola and finally into the baby's mouth. Food and Drug Administration website (www.fda.gov).  Used with permission.

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