Every four weeks the brain signals through hormones that it’s time for an egg cell to mature. This starts while you have your period.
Halfway your cycle the ovary ruptures and releases the egg cell. This is called ovulation. Some girls can feel this happening. It’s felt as a sharp twinge.
Halfway through your cycle the lining of the uterus thickens, preparing for the egg if it is fertilized. It becomes spongy, filled with blood.
When you don’t have sex or use contraception, the egg will not be fertilized by a sperm cell. The lining disintegrates and leaves the body through the vagina: the girl has her period.
A new cycle starts.