Breast development is one of the important features of female early adolescent sexual development, which usually starts to develop at the age of 9-13 as a normal time and gradually completes at the age of 13-15. However, due to the influence of individual physique, genetics, environment, diet and other factors, this age is not fixed, and there is a certain range before and after it, and the whole development process has a large time difference, but most of them are between 1.5-6 years, which can gradually complete the whole process of breast development and sexual maturity of girls. Under normal circumstances, in the initial period of breast development, that is, when girls are 9-10 years old, small hard nodes will begin to appear on the breasts, at this time the nipples and areolas will gradually become obvious and larger, and sparse, light-colored hair will appear on the pubic area; when girls are 10-12 years old, with the rapid growth of the ovaries, estrogen levels gradually rise, breasts and areolas gradually increase, while the color and extent of pubic hair appear to deepen and expand; by When girls are 12-13 years old, their breasts gradually form a hemispherical projection and gradually take shape, and at this time, the first menstrual period also comes, marking the gradual maturation of girls’ sexual development. However, if breast development occurs earlier than 8 years old or later than 15 years old and no breast development or other sexual characteristics appear, it is an abnormal situation. Patients and their families should pay attention to this and go to the hospital to check if there is a possible disease, and take timely measures to treat it.