This population-based cohort study showed decreased pain threshold and maximal pain tolerance in women with endometriosis in the late fertile age of 46 years. The pain was also found to be more bothersome and intense compared with controls.

Study question Do women with endometriosis have decreased fertility and decreased number of children compared with women without endometriosis in a population-based study setting? Summary answer Women with endometriosis have decreased fertility compared with women without endometriosis but give birth to at least one child as often as women without endometriosis. What is known already Endometriosis is known to be associated with infertility. It is estimated that 25-50% of patients undergoing fertility treatments have diagnosis of endometriosis. Up to date, population-based studies investigating the association of endometriosis and fertility are lacking. Study design, size, duration In a large follow-up national birth cohort (n = 5889), a postal questionnaire was sent to women at ages 31 and 46 years (response rates 76% and 65%, respectively). At age 46, the postal questionnaire included a question on previously diagnosed endometriosis (n = 261). Furthermore, the cohort was linked to Finnish national register for health care to get the register-based diagnosis for endometriosis (n = 257) and to the national birth register for all births from 1987 to 2014. Participants/materials, setting, methods The final study population included 319 women with endometriosis and 2714 women without endometriosis. Time to pregnancy and fertility treatments were inquired via questionnaire at ages 31 and 46 years. Number of deliveries and mother`s age at the birth of the first child was derived from national birth register between 1987-2014. Pearson’s Chi-square tests, Mann-WhitneyU test and Kaplan-Meier method (with Mantel-Cox test) for survival analysis were carried out using IBM SPSS Statistics 27. Main results and the role of chance At age 31 years, women with endometriosis had decreased fecundability as they reported more often more often over 12 months time to pregnancy (11.2% vs. 22.9%, p < 0.001) when compared with women without endometriosis. At ages 31 years and 46 years women with endometriosis reported higher need for fertility treatments (OR 2.80 [95%CI 1.35 – 5.81], p = 0.005 and 46yrs OR 2.82 [95% CI 1.80-4.43], p < 0.001). Survival analysis showed that according to the birth register by age 48 years, women with endometriosis had given birth to their first child at an older age (median 29.79yrs [95%CI 28.74 – 30.85] vs. 27.51yrs [95%CI 27.24 – 27.78], p < 0.001) and had fewer children (mean 2.29 vs 2.63, p = 0.001) when compared with women without endometriosis. However, women with endometriosis had at least one child as often as women without endometriosis (85.3% vs. 88.6%, p = 0.074) by age 48 years. Limitations, reasons for caution Fecundability was asked in the postal questionnaire only at age 31 years but not at 46 years. Infertility was evaluated based on self-report. Wider implications of the findings This unique, population-based data showed that fertility among women with endometriosis is compromised, as these women had decreased fecundability, increased need for fertility treatments and less children compared with the reference group. The awareness of the effects of endometriosis on fertility should be increased and met with adequate care. Trial registration number Not applicable