Girls can expect to see the beginnings of breast formation as they approach their adolescent years. Soon, the hormone estrogen causes fat to start accumulating in breasts and the duct system forms, which will allow a girl to feed a baby when she is an adult, according to the Ohio State Medical Center. Breasts develop in a similar pattern, but they don’t all look the same and don’t develop at the same time in all children.

Age of Onset

Girls develop breasts at different times based on a variety of factors such as:

  • heredity
  • nutrition
  • weight
  • level of activity
  • stress levels
  • according to the Center for Young Women’s Health at Children’s Hospital Boston 14

Some girls begin developing breasts at age 7 or 8 years, and others won’t see any development until they are 12 or 13 years old, says the Center for Young Women’s Health 14. A girl who is active in sports may not enter puberty until the later end of the range, and a girl who is overweight may begin to develop a little earlier.


The beginning stage of breast development is considered “pre-adolescent,” where only the nipple tip is raised from the skin, according to the Ohio State University Medical Center 1. During stage 2, the areola gets larger, and the breast “buds” appear, slightly raising the breast and nipple. The third stage of breast development is when a girl’s breasts become slightly larger and glandular breast tissue forms.

Early or Delayed Breast Development

If you notice breasts forming in your child before she is 7 or 8 years old, she may be going through what is known as “precocious puberty,” according to KidsHealth 23. Your child’s early development may be a sign of an underlying health condition, so talk to your pediatrician to rule out problems or to treat your child. If your child is older than 12 or 13 years of age, you may also want to seek guidance from your pediatrician. However, in many cases, delayed puberty occurs because your child has inherited the tendency to develop later than her peers, says KidsHealth 23.

Normal Variations

Some girls’ breasts develop at two different rates. This is normal, and most of the time, the difference in sizes diminishes toward the end of development, says the Center for Young Women’s Health 14. Also, no single breast size is ideal. Breast size among girls can vary based on factors such as genetics, weight and hormones, according to the Center for Young Women’s Health 14. Finally, other common traits on fully developed breasts are stretch marks and some hair around the areola.

Breast Development in Boys

A temporary imbalance in hormones can lead breast tissue to enlarge in boys as they go through puberty. This condition, known as gynecomastia, occurs in about 70 percent of boys and it is typically temporary, according to MassGeneral Hospital for Children. Gynecomastia generally appears as a hard lump smaller than 1 inch beneath one side of the chest. If you or your child notice changes in his breast tissue, mention the change on the next doctor visit. Chances are high that your child won’t need any extra treatment, but he may need surgery if the breast is large or the tissue doesn’t recede within a couple of years, according to MassGeneral Hospital.