The true prevalence of endometriosis remains undetermined because diagnostic laparoscopy cannot be performed on large, random, population-based samples of women. However, available evidence suggests that the disease occurs in approximately 1% to 7% of women in the United States. The cause of endometriosis is though to be associated with mechanical factors (such as retrograde menstrual flow) or estrogen production. Recent research also has suggested that regular exercise may help to protect against the disease by decreasing the rate of estrogen production.