A period is a release of blood from a girl's uterus, out through her vagina. 
It is a sign that she is getting close to the end of puberty. 
It also means that she can get pregnant if she has sex.
There is a lot to learn about periods. Here are answers to some common questions.
When Do Periods Usually Start?
Most girls get their first period when they're around 12. 
But sometimes it comes a few years earlier or later. 
Every girl's body has its own schedule.
There isn't one right age for a girl to get her period. 
But signs to watch for include:
breasts starting to develop (periods usually start about 2 years after this), hair growing under a girl's arms and in her private parts, seeing or feeling vaginal discharge fluid (sort of like mucus), which usually begins about 6 months to a year before a girl gets her first period
What Causes a Period?
A period happens because of changes in hormones in the body. 
Hormones are chemical messengers. 
The ovaries release the female hormones estrogen and progesterone , which cause the lining of the uterus (or womb) to build up. 
The built-up lining is ready for a fertilized egg (an egg that has joined with a sperm) to attach to and develop into a baby. 
If there is no fertilized egg, the lining breaks down and bleeds. 
Then the same process happens all over again.