Menstrual characteristics associated with endometriosis.

This case-controlled study investigates whether the presence of menstrual characteristics, which may increase the exposure of the peritoneal cavity to retrograde menstruation, increases the risk of developing endometriosis. The menstrual characteristics considered were 1. age at menarche of less than 12 years, 2. duration of menstrual flow greater than five days and 3. menstrual cycle lengths of < 28 days. The frequency with which these menstrual characteristics occurred in 305 women with proven endometriosis was compared with their frequency in 305 age-matched women without endometriosis. Adjusting for the confounding factors of parity, age at first childbirth and social class, the only menstrual characteristic that was significantly associated with endometriosis was menstrual cycle lengths of less than 28 days (odds ratio 1.83; 95% confidence intervals 1.60-2.55). There was insufficient evidence to conclude that the presence of menstrual characteristics which may increase the exposure of the peritoneal cavity to retrograde menstruation, increase a women's risk of developing endometriosis. The association of short menstrual cycle lengths with endometriosis may have been consequential rather than causal.