The time between breast development and menstruation is about 2 to 2.5 years, but the time between breast development and menstruation may be shortened or lengthened if the child’s sexuality develops earlier. Usually, a girl’s breasts start to develop around the age of 11, and she will have her period around the age of 13 to 13½, so the time between the onset of breast development and the onset of menstruation is about 2 years to 2½ years. However, some girls may have precocious puberty or early sexual development, for example, their breasts may start to develop at around 8 years of age, in which case the hormone level in the patient’s body is usually unstable, and the time between the onset of breast development and the onset of menstruation may be shortened or lengthened, which may be less than or more than two and a half years. Parents are advised to pay attention to their children’s physical development, and if there is any abnormality, they should go to the regular hospital for consultation in time.