Breasts vary in both size and shape, and their external appearance is not predictive of their internal anatomy or lactation potential. The shape of a womans breasts is in large part dependent on their support, which primarily comes from the Cooper's ligaments, and the underlying chest on which they rest. The breast is attached at its base to the chest wall by the deep fascia over the pectoral muscles.

On its upper surface it is given some support by the covering skin where it continues on to the upper chest wall. It is this support which determines the shape of the breasts. In a small fraction of women, the frontal milk sinuses (ampulla) in the breasts are not flush with the surrounding breast tissue, which causes the sinus area to visibly bulge outward.

The development of a girl's breasts during puberty is triggered by sex hormones, chiefly estrogen. This hormone has been demonstrated to cause the development of woman-like, enlarged breasts in men, a condition called gynecomastia, and is sometimes used deliberately for this effect in transwomen who receive hormone replacement therapy.

In most cases, the breasts fold down over the chest wall during Tanner stage development, as shown in this diagram. It is typical for a womans breasts to be unequal in size particularly while the breasts are developing. Statistically it is slightly more common for the left breast to be the larger. In rare cases, the breasts may be significantly different in size, or one breast may fail to develop entirely.

A large number of medical conditions are known to cause abnormal development of the breasts during puberty. Virginal breast hypertrophy is a condition which involves excessive growth of the breasts, and in some cases the continued growth beyond the usual pubescent age. Breast hypoplasia is a condition where one or both breasts fail to develop.

In Cameroon, some girls are subjected to breast ironing to stunt breast growth in order to make them less sexually attractive in the belief that this makes them less likely to become a victim of rape.

As breasts are mostly composed of adipose tissue, their size can change over time. This occurs for a number of reasons, most obviously when a girl grows during puberty and when a woman becomes pregnant. The breast size may also change if she gains (or loses) weight for any other reason. Any rapid increase in size of the breasts can result in the appearance of stretchmarks.

It is typical for a number of other changes to occur during pregnancy: in addition to becoming larger, the breasts generally become firmer, mainly due to hypertrophy of the mammary gland in response to the hormone prolactin.

The size of the nipples may increase noticeably and their pigmentation may become darker. These changes may continue during breastfeeding. The breasts generally revert to approximately their previous size after pregnancy, although there may be some increased sagging and stretchmarks.

The size of a womans breasts usually fluctuates during the menstrual cycle, particularly with premenstrual water retention. An increase in breast size is a common side effect of use of the combined oral contraceptive pill.

The breasts naturally sag through aging, as the ligaments become elongated.

Skin covering the breasts has the same healing ability as other skin and can take much of the activity involved in breast torture without long-term effects. The nipples and areola were designed for suckling and can also take a certain amount of harm. Skin blistering and fatigue are reported almost immediately. After continued exposure, the skin may darken, thicken and lose sensitivity due to nerve endings being damaged. Skinny and large breasted is not a natural combination.