A girl’s first menstruation at the age of 10-15 is in the normal range. If you are more than 15 years old and have not had your period, you should consider primary amenorrhea, or if you are more than 14 years old and have not had your period, and the secondary female sexual characteristics have not yet begun to develop, you should consider primary amenorrhea. Most women have their first menstrual period at the age of 11-12. The first menstrual period in women is called menarche and is an important sign of puberty. The first menstrual period is on average 2.5 years later than breast development. The onset of menstruation suggests that the ovaries produce enough estrogen to proliferate the endometrium, and menstruation occurs when estrogen reaches a certain level and fluctuates significantly, causing the endometrium to shed. During the first few years of menstruation, adolescent girls are not able to ovulate even if the follicles are mature because the central positive feedback mechanism to estrogen is not yet mature, so the menstrual cycle is prone to irregularity. After 5-7 years of establishing regular periodic ovulation, menstruation can gradually become normal.