Menstruation is one part of a girl’s reproductive cycle. Each month, an egg (ovum) matures and pops out of one of a girl’s two ovaries. This is called ovulation. (A girl’s ovaries are located inside her abdomen. They contain all the eggs (ova) she’ll ever have that could become fertilized by a guy’s sperm.) Girls usually start ovulating during puberty.

The egg travels from a girl’s ovary, through one of her fallopian tubes toward her uterus. As the egg travels, it sends a signal to her body to develop a thick lining of blood and tissue inside the uterus. The job of this lining is to help the egg attach to her uterus, should it meet up with a sperm and become fertilized. If this happens, the fertilized egg latches on to the thick lining of tissue and blood, and a pregnancy begins.

Now, if there’s no fertilization by sperm, there’s no pregnancy and no need for that thick lining to nourish a pregnancy. A girl has her period or is menstruating when the blood and tissue that line the uterus slowly pass out of the body through the vagina over five to seven days.

Check out “The Menstrual Cycle,” a cool animation of a girl’s menstrual cycle.