First birth and total fertility rate in women with surgically verified endometriosis - A nationwide register study of 18 320 women across reproductive life course.

Introduction: Endometriosis is associated with pain and infertility. However, little is known about birth rate among women with endometriosis on population level. We studied whether women with endometriosis have lower birth rate than women in the general population.

Material and methods: This historical population-based cohort study used data from 18 320 fertile-aged women with first surgical verification of endometriosis in 1998-2012. Women with endometriosis were further divided into sub-cohorts: women with solely peritoneal (n = 5786), ovarian (n = 6519) and deep endometriosis (n = 1267). Women with combined types and rare forms of endometriosis formed a sub-cohort of combined/other endometriosis (n = 4748). The reference cohort comprised 35 788 women. The follow-up started at the age of 15 years and ended at first birth, sterilization/bilateral oophorectomy/hysterectomy, emigration, death, age of 50 years, or December 31, 2019. From Kaplan-Meier survival curves of not giving birth, that is, until the first birth, we assessed the statistical difference of first births with crude and adjusted restricted mean survival time (RMST). In addition, we studied the fertility rate of women until the end of follow-up.

Results: Altogether 12 491 (68.2%) women with endometriosis compared with 28 871 (80.7%) reference women gave birth during follow-up. Women with peritoneal and deep endometriosis had higher first birth rate (73.1% and 71.3%) compared with women with ovarian and combined/other forms of endometriosis (65.2% and 65.5%) (p < 0.001). The RMST of not giving birth was longer in women with endometriosis 18.9 (18.8-19.0) years compared with the reference cohort 15.5 (15.4-15.6) with both crude and adjusted RMST difference (p < 0.001). Moreover, each sub-cohort showed a longer RMST of not giving birth than reference cohort (p < 0.001). Total fertility rate of women was 1.33 (SD 1.16) in the endometriosis and 1.89 (1.46) in the reference cohort (p < 0.001) with smaller differences among endometriosis sub-cohorts.

Conclusions: Findings suggest that fertility outcome is compromised depending on the endometriosis subtype. Thus, timely diagnosis and appropriate treatment might be beneficial for fertility.