Puberty and Menstruation
The most obvious cause of breast enlargement is puberty. During this period of a girl's life, the ovaries start to produce estrogen. This hormone causes fat to accumulate in the breasts. The accumulating fat causes the breasts to grow outwards until they reach their adult size. After a girl starts having her periods, she may experience temporary breast swelling during the times of the month right before she begins to menstruate. This short-term growth is caused by glands in the breasts growing bigger in case of pregnancy. Once menstruation begins, the breasts return to their normal size.
Contraception and Pregnancy
During pregnancy, the hormone progesterone readies the breasts to produce milk. The breasts grow larger due to additional fat and swollen milk glands. The breasts stay enlarged while the mother breastfeeds her infant. Once the baby is weaned off breast milk, the breasts often return to their original size or may even become smaller. Using a hormonal method of contraception, such as the birth control pill, can also make breasts larger as such methods contain hormones that mimic the effects of pregnancy. The side effect of fluid retention can also swell the breasts.
Weight Gain and Exercise
Since the breasts contain fat, if a woman gains weight in the form of fatty tissue, her breasts might grow bigger as well. Exercise can decrease breast size by burning off fatty tissue. Exercising the pectoral muscles underneath the breasts can counteract the lost fatty breast tissue. Larger pectoral muscles give the illusion of larger breasts. Strong pectorals can also make the breasts appear firmer. Push-ups, chest presses, chest flies and incline presses are some of the many exercises that can help strengthen the pectorals.