Every month or so, the uterus lining gets thicker to prepare for a fertilized egg if the woman becomes pregnent. If the egg doesn't get fertilized, the lining is released from the body as blood through the vagina. This monthly process is called mensruation.
Why do girls get periods?
The onset of puberty varies among individuals. Puberty usually occurs in girls between the ages of 10 and 14, while in boys it generally occurs later, between the ages of 12 and 16. In some African-American girls, puberty begins earlier at the about the age of 9 meaning that puberty occurs between 9 and 14.
What age does puberty start at?