The onset of menstruation usually occurs 1-2 years after the development of breasts, and if a girl of 9 years of age develops a breast nucleus, she will therefore menstruate around the age of 11 years. The first menstrual period is called menarche, and it is an important sign that a woman is entering puberty. The first menstrual period usually occurs on average 2.5 years after breast development, and menstruation may usually occur 1-2 years after breast development. In recent years, due to changes in diet and living habits, the age of menarche has advanced. Due to the central positive feedback mechanism of estrogen in puberty is not yet mature, even if the follicle is mature, it can not ovulate, so the menstrual cycle is not regular, after 5-7 years to establish a regular cycle of ovulation before menstruation is gradually normal. The development of breast nucleus in 9-year-old girls, pay close attention to the first menstruation, if more than 16 years old menstruation is still not the first menstruation, should be highly cause for attention.