The medical term for menstrual pain is dysmenorrhea. Cramps are usually felt in the pelvic area and lower abdomen, but can radiate to the lower back or down the legs. Many girls have cramps severe enough to keep them home from school. Dysmenorrhea is the most frequent cause of absenteeism from school among younger women. Women seem to go through phases when cramps are severe, then get better for several years, and then maybe worsen again.
Most women find they have less menstrual pain after having children.
Mechanically, cramps are like labor pains. Often, after several years of menstruating or after childbirth, the cervical opening enlarges. The uterus doesn't have to contract as much to discharge the menstrual flow, so there is less cramping. Menstrual pain may also come from the bleeding process itself. When the uterine lining separates from the wall, it releases chemicals called prostaglandins.