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

Abstract

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. Lay summary Embryo transfer is the last step in the IVF process. The position of the transferred embryo in the uterine cavity may affect the clinical pregnancy rate. The relationship between the position of the embryo in the uterine cavity at the time of transfer and the clinical pregnancy rate is disputed. In this study, the distance from the air bubble to endometrial cavity fundus measured using the freeze-frame ultrasound immediately after embryo transfer was used to indicate the position of the embryo in the uterine cavity. This study demonstrates that the position of the transferred air bubble in the uterine cavity is an independent factor on clinical pregnancy rate in blastocysts (embryos on the fifth day of fertilization) frozen-thawed embryo transfer cycles but not in cleavage embryos (embryos on the third day of fertilization) frozen-thawed embryo transfer cycles. The distance from the air bubble to endometrial cavity fundus between 3 mm and 15 mm is the optimal position in blastocyst frozen-thawed embryo transfer cycles.