Rotating nightshift work and the risk of endometriosis in premenopausal women

OBJECTIVE: The purpose of this prospective study was to study the association between rotating nightshift work and endometriosis risk within the Nurses' Health Study II. STUDY DESIGN: We found 89,400 women without diagnosed endometriosis at baseline; the 2062 laparoscopically confirmed cases that were documented during 16 years of follow-up evaluation formed our study population. RESULTS: Overall, there was no association between rotating nightshift work and risk of endometriosis. When the cases were categorized by infertility status, risk was elevated among women with concurrent infertility and >= 5 years of rotating nightshift work (rate ratio, 1.71; 95% confidence interval, 1.18-2.49; P-trend = .005), compared with women with no rotating nightshift work. In contrast, there was no association among women without reported infertility (P-heterogeneity = .003). CONCLUSION: Women who work rotating nightshifts for >= 5 years may have a modestly elevated risk of endometriosis if concurrently infertile. However, the relation between shiftwork, endometriosis, and infertility is complex, and the potential for bias because of a healthy or infertile worker effect must be considered.