Why do girls get periods?

There are 2 ovaries in a girl's body and each month these [b]ovar[/b]ies (ovar not under!) fight very hard to see who's the best. so they get hurt and bleed!!!
and when the girl have a baby in their tummy, the ovaries decide to help the baby and make it safe so they decide no fighting!! so no blood.
...and then in the future... when the girls get old, the ovaries get old too and ride in a wheelchair and don't have strength to fight anymore.