Correlating the Endometrial Thickness and Assisted Reproductive Technology Outcome in Women Who Underwent Frozen-thawed Embryo Transfer after Endometrial Preparation Using Hormone Replacement: A Retrospective Cohort Study

Aim and background: Synchronized endometrial thickness and embryonic development is a requisite for successful assisted reproductive technology (ART) outcome.This study aimed to correlate endometrial thickness on the day of progesterone initiation with ART outcomes in women undergoing frozen-thawed transfer of good quality cleavage-stage (day 3) and blastocyst-stage (day 5) embryos after hormone-replaced endometrial preparation.Materials and methods: This retrospective analysis included records of 252 patients who satisfied the inclusion criteria.The cohort was divided into day 3 frozen-thawed embryo transfer (FET) cycles (n = 171) and day 5 FET cycles (n = 81).Within each category, patients were grouped by endometrial thickness: Group I (<7 mm), group II (7-9 mm), and group III (10 mm).Data were analyzed using the Chi-square test.Results: In day 3 FET cycles, pregnancy rates were significantly higher in group III when compared to group II (53 vs 24%; p = 0.0027).For day 5 FET cycles, pregnancy rates were 50% for group II and 46% for group III, with no statistically significant difference observed between the groups.Notably, pregnancies were achieved even with an endometrial thickness of 6.3 mm.There was no significant difference in the secondary outcomes across groups for either day 3 or 5 transfers. Conclusion:Our study demonstrates a positive correlation between endometrial thickness and pregnancy rate in day 3 embryo transfers (ETs), but not with day 5 ETs.The achievement of pregnancies with thinner endometrium suggests that endometrial thickness may not be the sole determinant of ART success, highlighting the potential importance of other factors such as endometrial pattern.Clinical significance: Our study findings state that thicker endometrium helps to increase the pregnancy rates on transferring a day 3 embryo.Whereas, occurrence of pregnancies even with a thin endometrium indicates the importance of another factor which is the endometrial pattern.