Breasts are the only external human organs whose shapes and sizes change so frequently. Even during the monthly menstrual cycle of the female, there are marked changes in the bulk and texture of the breasts. This is an ongoing process. The breasts change their shapes through the cycle and then return to almost as they were when the cycle ends. In most women, the changes are unnoticeable, but some women can clearly see the changes as their month advances.

The menstrual period is only one of the factors that affect the sizes and shapes of the female breasts. There are many more such periods in a woman's life. Changes are seen in the breasts when the woman loses her virginity, when the woman becomes pregnant, all through the pregnancy, during the lactation, when the lactation stops, during menopause and finally when the woman reaches her old age. As the woman grows, the breasts also grow along with the woman. If we were to plot a graph of the woman's growth with her breasts' growth, then there would be obvious similarities to be seen.

The first changes in the breasts are seen when the girl reaches her puberty, which could be anytime after eight years. Actually this is when the girl actually gets breasts, for they were just a pair of nipples on the chest until that time. With puberty, hormones like the estrogen trigger the growth of the breasts. The breasts begin to become rounder in shape, the nipples become more prominent and the areolas widen and darken.

Breasts keep on developing all through the puberty. They never actually settle into one final shape until the woman becomes pregnant for the first time. This is when the pubertal changes come to an end and the changes due to pregnancy begin to take place. During pregnancy the breasts tend to become bulkier and rounder. Hormones need to trigger the breasts to get ready for the lactation that is to come. So in preparation, the breasts begin to attain their maximum possible size. Just a couple of days before childbirth, the areolas could become wrinkled and the milk may begin to ooze out of them. Lactation gives breasts their most functional shape. The breasts become tauter so that the nipples could be easily accessible to the infants. There is also an increase in the length of the nipples due to the infant continually suckling on them.