The Ohio State University Medical Center's information on breast development points out that women develop breasts long before they are needed to nurse a baby, a characteristic that is unique to mammals. Although the beginnings of the milk duct system are present at birth, most breast development occurs over a four-year span during puberty.
Physiology
For up to 92 percent of Caucasian girls, development of the breast signals the onset of puberty, says the Massachusetts General Hospital. Breasts grow and change under the influence of the female hormone estrogen, which first deposits fat around the breasts. As development continues, the milk ducts grow and the secretory glands at the ends of the ducts form. All of these changes prepare the duct work of the breasts for the eventual production and secretion of milk.
Stages
Doctors classify pubertal breast development in girls into four stages using the Tanner scale. In stage one, which happens in preadolescence, only the nipple is raised. In stage two, generally considered the start of puberty, breast buds appear. By stage three, glandular tissue develops, and in stage four the areole and nipple sit on a separate mound above the mound of the breast. In the final stage, the adult breast is rounded and only the nipple is raised.
Time Frame
Breast development typically begins in girls between the ages of 9 and 13 and takes about four years to complete. On average, African-American girls tend to have earlier breast development than Caucasian girls. Breast development in girls younger than 8 is considered precocious (early) puberty and warrants a medical examination, says the Massachusetts General Hospital. Likewise, a girl whose breast development has not started by age 13 should see a doctor.
Concerns
Girls whose breasts do not develop might have a chronic illness, or they might lack the underlying gland tissue, which is rare. According to the Massachusetts General Hospital, up to two percent of women develop extra nipples or extra breasts. Although these are harmless, some women chose to have them removed. During puberty, breasts often develop asymmetrically. Although this typically resolves, up to 25 percent of women have noticeably different-sized breasts, says the Massachusetts General Hospital. This condition should be checked by a doctor. Unusually large breasts can cause backaches or even curvature of the upper spine called kyphosis.
Males
The Massachusetts General Hospital says that about 70 percent of boys develop some breast tissue during puberty due to a temporary imbalance of hormones. This condition, called gynecomastia, generally recedes on its own within about two years. Using marijuana and other illicit drugs can worsen gynecomastia.
