Girls usually start menstruating between the ages of 11 and 15. Due to individual differences, the timing of menstruation varies. Girls living in urban areas start menstruating earlier, usually between the ages of 12 and 15. Girls living in rural areas start menstruating later, usually between the ages of 13 and 17. If a girl menstruates before the age of 9, she is clinically diagnosed as having precocious puberty, so she should go to the hospital for intervention and treatment. If the girl has not menstruated beyond the age of 18, she must go to the hospital for a detailed examination to rule out the possibility of congenital absence of the uterus or uterine developmental abnormalities, and clinical intervention should be provided as soon as possible.