Each month, the inner lining of the uterus (the endometrium) normally builds up in preparation for a possible pregnancy. After ovulation, if the egg is not fertilized by a sperm, no pregnancy will result and the current lining of the uterus is no longer needed. The woman's estrogen and progesterone hormone levels decline, and the lining of the uterus becomes swollen and is eventually shed as the menstrual flow and is replaced by a new lining in the next monthly cycle.
Menstrual cramps are caused by the uterine contractions that occur in response to prostaglandins and other chemicals. The cramping sensation is intensified when clots or pieces of bloody tissue from the lining of the uterus pass through the cervix, especially if a woman's cervical canal is narrow.
a woman's menstrual cramps are too severe to be managed by these strategies, her doctor might prescribe low doses of birth control pills (oral contraceptives) containing estrogen and progestin in a regular or extended cycle. This type of approach can prevent ovulation (the monthly release of an egg) and reduce the production of prostaglandins which, in turn, reduces the severity of cramping and causes a light menstrual flow.