Oral contraceptive use and risk of endometriosis. Italian Endometriosis Study Group.

Objective. To analyse the association between use of oral contraception and risk of pelvic endometriosis. Design. We compared use of oral contraception in women with and without endometriosis. Participants. Eligible for the study were women with primary or secondary infertility (n = 393) or chronic pelvic pain (n = 424), requiring laparoscopy, consecutively observed between September 1995 and January 1996 in 15 obstetrics and gynaecology departments in Italy. Results. Out of the 817 women included in the study, 345 had a diagnosis of endometriosis; 164 (47.5%) women with endometriosis and 139 (29.4%) without the disease reported ever using oral contraception. In comparison with never users the estimated odds ratios (OR) of endometriosis were 1.8 (95% CI 1.0-3.3) in current users and 1.6 (95% CI 1.1-2.4) in ex-users. No clear relation emerged between duration of oral contraceptive use and risk of endometriosis. In comparison with never users, the OR was 1.8 (95% CI 1.1-3.0) for women reporting their last use of oral contraception < 5 years before interview and 1.5 (95% CI 0.9-2.5) for those reporting their last use ≥ 5 years before interview. Conclusions. The study suggests that oral contraception is associated with an increased risk of endometriosis but this finding is based on a selected population and cannot generalised to all women with endometriosis.