At the age of 12, girls have almost reached puberty. At this age, secondary sexual characteristics such as breasts begin to develop gradually. Under normal circumstances, there is no fixed standard value for the chest circumference of 12-year-old girls, which is around 60cm, and there is a certain relationship between chest circumference and height. In addition, each girl may enter puberty at a different time. 12-year-old girl’s breast circumference also has a certain relationship with the girl’s development status, nutrition, physical condition, personal fat and thin, estrogen level in the body and genetic factors.