Question

what is menarche and why

Answer 1

The first observation of menstruation is called menarche.

Answer 2

Menarche is the first menstrual flow in girls. It starts at the age of 9 - 12 years. It happens because the first egg is released at that age, but if the girl don't do sexual act and the egg does not get fertilized. Then inner lining of uterus alongwith blood shed off. This causes bleeding. However, if the girl do a sexual act and the egg get fertlized with the sperm, then there would be no bleeding.