A period happens because of changes in hormones in the body. Hormones give messages to the body. These hormones cause the lining of the uterus (or womb) to build up. This gets the uterus ready for an egg (from the mom) and sperm (from the dad) to attach and grow into a baby.

Are periods necessary?

Today, any doctor will tell you there is no medical necessity for periods unless you're trying to conceive. The body preps for pregnancy by thickening the uterus's lining, like a bird building a nest for her eggs; hormonal birth control prevents pregnancy, in part, by keeping the uterine lining from ever building up.

How does a period begin?

The 1st day of your menstrual cycle starts on the 1st day of your period (AKA menstruation). During your period, blood and tissue from the lining of your uterus flows out of your vagina. If you get your period, it means you didn't get pregnant during your last cycle.