Reproductive and menstrual factors and risk of peritoneal and ovarian endometriosis

Objective: Between 1987 and 1989 data were collected to evaluate risk factors for pelvic endometriosis. Design: A case-control study was conducted on 241 cases with laparoscopically or laparotomically confirmed peritoneal or ovarian endometriosis consecutively admitted to three teaching hospitals in Northern Italy. The control group consisted of 437 women admitted to hospitals for acute conditions covering similar catchment areas. Results: Compared with nulliparous women, the risk of endometriosis decreased with increasing number of births: the point estimates were 0.4 and 0.3, respectively, for those with one and two or more births (X²/₁ trend = 50.3, P <0.001). No relation emerged with age at first birth and spontaneous miscarriages. Relative to women whose menarche occurred at age 11 or younger, the risk of endometriosis was slightly lower in those who experienced later menarche, but the trend in risk was not significant. Women with irregular menstrual cycles showed a lower frequency of the disease (relative risk, 0.3; 95% confidence interval, 0.2 to 0.5). The role of various factors was largely similar for different disease locations (ovary, peritoneum, and both) and indication for diagnostic surgery (sterility, pelvic pain, and other reasons). Conclusions: This study found that parity and irregular/long menses lower the risk of endometriosis. These findings were similar in different subgroups of disease location and indication for surgery, giving strong evidence of the consistency of the general results.