Length of Menstrual Cycle and Risk of Endometriosis: A Meta-Analysis of 11 Case–Control Studies

Endometriosis is a complex disease that affects a large number of women worldwide and may cause pain and infertility. To systematically review published studies evaluating the relationship between menstrual cycle length and risk of endometriosis. We searched the Cochrane Library, PubMed, Web of Science, and EMBASE in databases in July 2014 using the keywords "case-control studies," "epidemiologic determinants," "risk factors," "menstrual cycle," "menstrual length," "menstrual character," and "endometriosis." We included case-control studies published in English that investigated cases of surgically confirmed endometriosis and examined the relationship between endometriosis risk and menstrual cycle. Eleven articles that met the inclusion criteria included data of 3392 women with endometriosis and 5006 controls. Fixed-effects and random-effects models were used for the evaluation. For the association of risk of endometriosis and menstrual cycle length shorter than or equal to 27 days (SEQ27) or length longer than or equal to 29 days (LEQ29), the odds ratio was 1.22 (95% confidence interval [CI]: 1.05-1.43) and 0.68 (95% CI: 0.48-0.96), respectively. In conclusion, this meta-analysis suggests that menstrual cycle length SEQ27 increase the risk of endometriosis and cycle length LEQ29 decrease the risk.