Many physicians believe the breasts are not fully mature until a woman has given birth and produced milk. Breast changes are one of the earliest signs of pregnancy - a result of the pregnancy hormone, progesterone. In addition, the areolas (the dark areas of skin that surround the nipples of the breasts) begin to swell followed by the rapid swelling of the breasts themselves. Most pregnant women experience tenderness down the sides of the breasts and tingling or soreness of the nipples because of the growth of the milk duct system and the formation of the many more lobules.
By the fifth or sixth month of pregnancy, the breasts are fully capable of producing milk. As in puberty, estrogen controls the growth of the ducts and progesterone controls the growth of the glandular buds. Many other hormones, such as follicle stimulating hormone (FSH), luteinizing hormone (LH), prolactin, oxytocin, and human placental lactogen (HPL) also play vital roles in milk production.
Other physical changes, such as the prominence of the blood vessels in the breast and the enlargement and darkening of the areola occur. All of these changes are in preparation for breastfeeding the baby after birth.