Materials and Methods: This prospective cohort study was performed in a university-affiliated fertility clinic from March 2020 to March 2021. The eligible women undergoing their first or second FET cycle and having the top grading cleavage stage embryos were enrolled. All patients received the same endometrial preparation regime. The alteration in endometrial thickness (EMT) between the day of progesterone initiation and the day of embryo transfer (ET) was measured using consecutive transvaginal sonography. The patients were divided into three groups based on the percentage of endometrial compaction (i.e., the difference of EMT at end of the estrogen-only phase and after three days of progesterone administration (ET day) divided by the EMT on the terminal day of the estrogen-only exposure).

Results: Overall, 300 eligible women were evaluated and only 27.3% (82/300) of the studied cycles showed ≥5% compaction, whereas 72.6% (218/300) either expanded or showed minimal compaction. The clinical and ongoing pregnancy rates in group 2 (any expansion) were significantly higher than those in groups 1 and 3 (P = 0.002 and P = 0.01, respectively). Multivariable logistic regression test indicated that the cycles with any expansion in ET were independently associated with 3.1 times improvement in clinical pregnancy rate in comparison to those with any compaction (P = 0.002).

Conclusion: Gross endometrial compaction occurred in one-third of FET cycles with no significant positive effect on pregnancy outcomes after cleavage-stage ET.