At puberty, the female breast develops, under the influence of estrogen, progesterone, growth hormone, prolactin, insulin and probably thyroid hormone, parathyroid hormone and cortisol. This complex process typically begins between ages 8 to 14 and spans about 4 years.

The breast contains mostly fat tissue, connective tissue, and glands that following pregnancy, will produce milk. The milk is collected in the ducts and transported to 15-25 openings through the nipple.

During the menstrual cycle, the breast is smallest on days 4-7, and then begins to enlarge, under the influence of estrogen and later progesterone and prolactin.

Maximum breast size occurs just prior to the onset of menses.

The breast is not round, but has a "tail" of breast tissue extending up into the axilla (or armpit).

This is clinically significant because abnormalities can arise there just as they can in other areas of the breast. During breast examinations, this area should be palpated.

Supernumerary breasts are relatively common. They are found along the "milk line," extending from the axilla to the groin.

Most of them are not noticed clinically until pregnancy occurs. Then, under the influence of the pregnancy hormones, the breasts enlarge in preparation for lactation. It is at this time that soft swellings along the milk line occur, representing supernumerary breasts. During lactation, the extra breasts may produce milk.

Although this image shows a supernumerary breast low in the abdomen, the two most common places for them are in the axilla and directly underneath the normal breast.

These are not dangerous and are generally ignored. If they prove to be a cosmetic problem, they can be removed surgically.

Supernumerary Nipples

More common than supernumerary breasts are supernumerary nipples. Like extra breasts, these are located in the milk line and are not dangerous.

Unless they are large, they are usually not noticed until a pregnancy. At that time, like the normal nipples, they enlarge and darken.

Inverted Nipples

Usually nipples point outward. Sometimes, they invert. When they persistently point inwards, they are called inverted nipples. This can be unilateral or bilateral.

With stimulation and nipple erection, most inverted nipples will evert. Occasionally, they remain inverted despite all efforts to evert them.

Other than for cosmetics, nipple inversion is not usually a problem. For breast-feeding, most inverted nipples will evert. Even those that do not evert may still function normally enough to allow for satisfactory infant nursing.