Female reproductive tract microbiota and recurrent pregnancy loss: a nested case-control study.

RESEARCH QUESTION: Is the composition of the endometrial or vaginal microbiota associated with recurrent pregnancy loss (RPL)? DESIGN: Endometrial and vaginal samples were collected from 47 women with two or more consecutive pregnancy losses and 39 healthy control women without a history of pregnancy loss, between March 2018 and December 2020 at Helsinki University Hospital, Helsinki, Finland. The compositions of the endometrial and vaginal microbiota, analysed using 16S rRNA gene amplicon sequencing, were compared between the RPL and control women, and between individual vaginal and endometrial samples. The mycobiota composition was analysed using internal transcribed spacer 1 amplicon sequencing for a descriptive summary. The models were adjusted for body mass index, age and parity. False discovery rate-corrected P-values (q-values) were used to define nominal statistical significance at q < 0.05. RESULTS: Lactobacillus crispatus was less abundant in the endometrial samples of women with RPL compared with controls (mean relative abundance 17.2% versus 45.6%, q = 0.04). Gardnerella vaginalis was more abundant in the RPL group than in controls in both endometrial (12.4% versus 5.8%, q < 0.001) and vaginal (8.7% versus 5.7%, q = 0.002) samples. The individual vaginal and endometrial microbial compositions correlated strongly (R = 0.85, P < 0.001). Fungi were detected in 22% of the endometrial and 36% of the vaginal samples. CONCLUSIONS: Dysbiosis of the reproductive tract microbiota is associated with RPL and may represent a novel risk factor for pregnancy losses.