What causes painful menstrual cramps?

What causes painful menstrual cramps?

Menstrual cramps happen when a chemical called prostaglandin makes the uterus contract (tighten up). The uterus, the muscular organ where a baby grows, contracts throughout your menstrual cycle. During menstruation, the uterus contracts more strongly. If the uterus contracts too strongly, it can press against nearby blood vessels, cutting off the supply of oxygen to muscle tissue. You feel pain when part of the muscle briefly loses its supply of oxygen.