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Each month the lining of the uterus (the endometrium) builds up in preparation for a possible pregnancy. If a pregnancy occurs, the fertilized egg attaches itself to the lining to be nourished as it develops into a baby. If the egg is not fertilized, the lining is not needed. It breaks down and hormones called prostaglandins are released. These trigger the muscles of the uterus to contract and squeeze the lining out. The muscles are the same ones that push a baby out during childbirth, so they are very strong. Some women may have higher levels of prostaglandins and this is thought to be what causes painful muscle spasms.
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