The impact of transferred air bubble position on clinical pregnancy rate in FET cycles

We aimed to investigate the effects of the position of the transferred air bubble with the clinical pregnancy rate (PR) in frozen-thawed embryo transfer (FET) cycles. A prospective clinical study was carried out at Reproductive Medicine Center of West China Second University Hospital between June 2020 and May 2021. A total of 1159 women who underwent FET were included in this study. Transabdominal ultrasonographic guidance was used during the transfer procedure. The distance from the air bubble to endometrial cavity fundus (DAF) was measured in the freeze-frame ultrasound immediately after ET. In DAF ≤3 mm, 3–15 mm, and ≥15 mm group, the clinical PR in women transferred with cleavage embryos was 33.3% (7/21), 55.0% (153/280), and 31.3% (5/16), respectively, and the difference was statistically significant (P < 0.05). Among women transferred with blastocysts, the clinical PR was 63.0% (34/54), 68.5% (485/708), and 55.0% (44/80), respectively, and the difference was statistically significant (P < 0.05). In multivariate logistic regression model for clinical PR, the clinical PR was associated with age, embryo quality, number of embryo transferred, and endometrial thickness. DAF was an independent risk factor influencing clinical PR in blastocyst FET cycles rather than in cleavage embryo FET cycles. In conclusion, our results suggested that DAF was associated with the clinical PR and DAF between 3 mm and 15 mm is the optimal position in blastocyst FET cycles.