Why do women have menstrual cramps?

During a menstrual period, the body releases chemicals called prostaglandins which stimulate muscles in the uterus to tighten and contract, causing pain in the lower abdomen, back, and thighs. While the pain is usually milder than the contractions felt during childbirth, for some women, menstrual cramps can be severe. Over-the-counter pain relievers and oral contraceptives can help reduce the pain associated with menstrual periods.