O-256 Effect of an endometrial microbiota on pregnancy outcome of frozen embryo transfer (FET) cycles
Authors: Nabeta M , Tanaka S.E , Note K , Hasegawa M , Sakai K , Arai W , Sakuraba Y , Iki A 
Published: June 2022

Abstract

Study question
Is specific microbiota in the endometrial fluid (EF) associated with pregnancy outcome in a frozen embryo transfer (FET) cycles?

Summary answer
Lactobacillus had high abundance in the microbiota of endometrial fluid of patients with successful pregnancy after FET treatment.

What is known already
Recently, the relationship between endometrial microbiota and repeated implantation failure (RIF) has been reported. While Lactobacillus-dominated microbiota (LDM, defined as &gt; = 90 % Lactobacillus species) in the EF of the receptive phase was reported to be associated with favorable reproductive outcome, non-LDM (&lt; 90 % Lactobacillus species) was found to decrease implantation, clinical pregnancy, ongoing pregnancy, and live birth rates. However, it is still unclear the effect of the microbiota on pregnancy outcome of the patients with the assisted reproductive technology, especially a frozen embryo transfer (FET) treatment.

Study design, size, duration
We included 802 cycles with clinical results of pregnancy outcome after FET treatment at our clinic from December 2018 to January 2021. Endometrium fluid was collected before FET and microbiota was examined. We examined the relationship between endometrial microbiota and pregnancy outcome in 463 cycles in which the endometrial microbiome test results were available, pregnancy outcome was known, CD138 negative, and less than 38years (229 cycles were positive pregnancy and 234 cycles were negative pregnancy).

Participants/materials, setting, methods
Sampling was performed carefully avoiding contamination before FET treatment. Extracted genomic DNA was sequenced for the region of 16S ribosomal RNA using next-generation sequencer (Endometrial microbiome test, Varinos, Japan). The sequencing data was assigned to bacterial taxonomy and the background-contaminated bacteria were excluded from the microbiome profile. Lactobacillus abundance was calculated. After concordance of patient background, Lactobacillus abundance between successful and unsuccessful pregnancy group was compared.

Main results and the role of chance
We investigated the effect of microbiota in EF, especially Lactobacillus, for the infertile female with frozen embryo transfer (FET) as an assisted reproductive treatment with large amount of cycles (802 cycles). First, all of cycles were divided into a successful pregnancy group (305/802) and an unsuccessful pregnant group (497/802), and then the abundance of Lactobacillus was analyzed (the pregnancy ratio was 38.0%). The mean abundance of Lactobacillus in EF was significantly higher in the successful pregnant group than in the unsuccessful pregnant group (70.2% 40.5 versus 63.5% 43.1, p = 0.007). To further refine the conditions of each cycle, patient backgrounds of each cycle were compared. There was a significant difference in age and CD138 results between the successful pregnant group and unsuccessful pregnant group, so the analysis was performed with the limited cycles with CD138-negative and under 38 years (229/463 versus 234/463, respectively). However, even under this strict condition, the mean abundance of Lactobacillus was significantly higher in the pregnant group than in the unsuccessful pregnant group (72.1% 39.7 versus 61.1% 44.4, p = 0.003). These results suggest that the endometrial environment with high Lactobacillus abundance is more conducive to pregnancy in FET treatment.

Limitations, reasons for caution
The limitation of this study is that the design is not a randomized controlled one, although it is prospective.

Wider implications of the findings
The results of this study suggest that the endometrial microbiota at the time of embryo implantation, especially Lactobacillus genus, is highly relevant to pregnancy outcome, implying that disruption of the intrauterine microbiota may cause infertility. We need further studies to improve the endometrial microbiota of infertile women.

Trial registration number
not applicable