The most common cause of chronic pelvic pain for women in the prime of their reproductive years is:

The most common cause of chronic pelvic pain for women in the prime of their reproductive years is endometriosis.

It means that the tissue that covers a woman's uterus grows outside of it, instead of covering its insides. This may cause infertility, as well as a lot of chronic pain, most of which occurs during menstruation and sexual activities. Although the cause is not known, and it cannot be cured, it can be treated with surgery, or taking painkillers.