Limitations and reasons for caution : Although women with undiagnosed endometriosis certainly remain in the comparison population even in this prospective cohort study, the community prevalence of endometriosis in an asymptomatic population is very low. moreover , the characteristics of this small proportion of undiagnosed cases are diluted among the > 90000 women accurately defined as being endometriosis -free and are , therefore , unlikely to impact on effect estimation. Although geographically diverse , the NHS 2 cohort is overwhelmingly caucasian, which may limit generalizability to more ethnically diverse populations.