Breast Development and Anatomy

Breast development begins in the womb with the formation of the milk ridge six weeks after conception. The milk ridge is a pairing of glands that run in two roughly parallel lines the length of the fetus. By the ninth week most of the glands have disappeared except for a single pair that will become the breasts. (Occasionally a woman may retain rudimentary nipples along her torso that can be removed by a plastic surgeon.)

After birth the breasts remain essentially unchanged until the onset of puberty. In most young girls the first sign of sexual maturation is the growth of breast buds, small areas of tissue that elevate the areolae (the pigmented circles surrounding the nipples). Breast bud development usually begins between the ages of 10 and 12. (The ages given here are general; each girl will mature at her own individual pace.)

As puberty advances the breasts continue to grow, the areolae increase in size, and pigmentation of the nipples may darken. It is not unusual for one breast to grow faster than the other, so adolescent girls should not be concerned if both breasts aren’t the same size or shape. Usually the breasts will nearly even out by the time breast development stops (ages 16 to 18), although it’s common for women to have one breast slightly larger than the other, just as one foot is often bigger than the other.

Breast development depends on a woman’s hormones—and her genes. “Breast size is genetically determined—it depends chiefly on the percentage of fat to other tissue in the breasts. Usually about a third of the breasts is composed of fat tissue. The rest is breast tissue,” says Susan M. Love, M.D., in her book Dr. Susan Love’s Breast Book, 2nd ed. (p. 3). Here’s Dr. Love’s description of the anatomy of the breast:

Inside, the breast is made up primarily of fat and breast tissue …The breast tissue is sandwiched between layers of fat, and behind the tissue and fat is the chest muscle. The fat has some give to it …The breast tissue is firm and rubbery …Included in the breast tissue are the all-important ducts and lobules. The lobules make the milk, and the ducts are the pipes that bring the milk to the nipple …The breast also has its share of the connective tissue that holds the entire body together. (pp. 5-6)

Because there is no muscle tissue in the breast, exercising cannot directly enlarge or firm the breasts. However, exercise can tone the pectoral muscles underlying the breasts so that those muscles provide more lift to the breasts.

Breasts continue to change throughout a woman’s life. Once she begins to menstruate, her breasts may swell each month as they prepare to produce milk. If conception does not occur, the breast enlargement subsides. Pregnancy and breastfeeding also temporarily enlarge the breasts. As a woman approaches menopause the reduction of estrogen in her body makes breast tissue shrink, and more fatty tissue grows within the breast. This is why older women’s breasts generally sag or droop more than younger women’s do. Women who take hormone replacement therapy after menopause will notice less of this effect than will women who don’t.

Finally, because fatty tissue composes a significant portion of the breast, breast size will increase or decrease with any significant weight gain or loss.