Hormone levels change throughout each month, causing the lining of the uterus to get thicker. When it gets thick enough, the lining sheds through the vagina. This is your menstrual period. It has blood and a lot of cells. Once your period ends, the lining grows thicker again and the cycle starts over.
People grow and develop at different rates.
Some girls will have their first period at 10, some at 16. Both can be normal. If you haven't had a period yet, and you're concerned, ask a parent, teacher, nurse or health care provider. During the first year or two, your period may be irregular. You may go several months without one.