When a female enters her teenage years, there is an increase in the levels of estrogen in the body. This is a hormone responsible for reproduction. The increase in this hormone causes the lining in your uterus to thicken, preparing you for eventual motherhood. This lining aids a fertilized egg to develop into a baby. If there is no egg to hold, your body will automatically start breaking down the lining and push it out of the uterus. The discharge of the lining from your uterus is what you see when you get your period. This cycle repeats once every 21 to 35 days. Your menstrual cycle may not begin at the same time every month because it follows your biological clock and it will take a few months to settle into a rhythm, especially when you first begin to get it.
Menstruation is a monthly cycle when blood and other impurities are discharged from your vagina. This normally last anything between 3 to 7 days.
You can expect to get your first period between the ages of 10 to 16. Do not panic if you don't get your period until you reach your 16th birthday. It is perfectly normal to have a delayed onset. Your body will respond appropriately when the time comes.