Teenage girls develop breasts years before they will need to produce milk for nursing. In fact, breast development starts in the womb, and by the time a baby enters the world, she already has a nipples and the beginning stages of her milk ducts. The most prominent changes, however, occur during the teenage years. It is during this time that a teen girl begins to see and feel that she is indeed becoming a woman.
The first sign of breast development is when puberty begins, which in girls is usually around the age of nine or 10. During this time, tiny lumps -- about the size of a nickel -- appear under each nipple. These are the breast buds. As the early teen years approach and a girl’s ovaries begin to release estrogen, fat accumulates and the breasts begin to grow larger. According to the Ohio State University Website, milk ducts also begin to grow at this time.
Breasts continue to develop throughout the teen years, with key changes occurring once a girl starts her monthly menstrual cycle. According to the Ohio State University website, with the onset of ovulation and menstruation, the milk ducts continue to grow and develop secretory glands at the end of each of the ducts. As the cycle continues, more fat accumulates within the tissue of the breasts, and both the areola and nipple are raised. By the time a girl reaches her late teens, the breasts are round, the areola is flat and the nipple remains upright.
As a teen girl’s breasts continue to develop, she might experience soreness at times. She might also notice that one breast begins to grow before the other, and in some cases, one breast is larger than the other. Sizes usually even out over time, but it is not unusual for an adult woman’s breasts to be slightly asymmetrical. According to the University of Hawaii System, this difference in breast size is normal throughout the development process, which takes between three and five years to complete.
Delayed Sexual Development
Girls who do not develop breasts by the age of 13, or those who do not have a menstruation cycle for five years or more after the first signs of breast formation, might be experiencing delayed sexual development. According to the NYU Langone Medical Center Department of Pediatrics website, the causes of delayed sexual development vary. While some girls are just late bloomers, chronic illnesses such as anemia, diabetes mellitus and cystic fibrosis might be the culprit in others. If you are a parent with concerns about your daughter's breast development, ask your family health provider for her opinion.