Sooner or later after childbirth, your menstrual cycle will resume. But here's a welcome side effect: You may have fewer bothersome cramps. Some women even find that menstrual pain ceases altogether after pregnancy and childbirth. This pain reduction is a well-known phenomenon, but no one really knows for sure why it occurs. One possible theory is that childbirth eliminates some of the prostaglandin receptor sites in the uterus. Prostaglandins are hormones with several functions, one of which is to direct the uterus to contract during labor. They also play a role in menstrual pain. The upshot? Fewer pain-receptor sites, fewer cramps.