This is called ovulation. The egg moves through one of two Fallopian tubes toward the uterus. Before the egg is let go, the uterus begins building up a lining made of tissue and blood, which becomes like a nest for the egg if pregnancy happens. Usually, the egg breaks apart before it gets to the uterus. But if a sperm from a boy's or man's penis meets the egg on its way to the uterus, it can fertilize the egg.
Pregnancy begins if a fertilized egg attaches itself to the lining of the uterus.
When the egg breaks apart, the lining of tissue and blood isn't needed. It flows out of the uterus, through the cervix , through the vagina , and out of the body. This is called menstrual flow. To keep the menstrual flow from getting on your clothes, you will want to use sanitary napkins or tampons. Some girls and women have their period only a few times a year.