Supplementary Material for: The impact endometrial compaction after progesterone administration on pregnancy outcomes in artificial frozen thawed embryo transfer cycles: A prospective cohort study

Objectives: The primary aim of this study was to association between endometrial compaction and pregnancy outcomes in HRT-FET protocols. The secondary objective was to investigate the correlation between serum progesterone levels and pregnancy success in FET cycles. Design: Prospective cohort study was conducted January- April 2024 Setting: A university-affiliated hospital, Tehran, Iran Participants: Infertile women under 40 years undergoing Hormone Replacement Therapy-Frozen Embryo Transfer (HRT-FET). Patients with uterine and genetic factors were excluded. Materials and Method: Data on endometrial thickness and serum progesterone levels were gathered, and patients were divided into groups based on the presence or absence of endometrial compaction using transvaginal sonography. Univariate and multivariate statistical analyses, including logistic regression models, were employed to assess factors associated with pregnancy outcomes. Results: Ninety participants with similar demographic profiles were enrolled in the study. The biochemical and clinical pregnancy rates were higher in the group with endometrial compaction, at 37.3% and 33.3%, respectively, compared to 15.4% in both categories for the group without compaction, with a statistically significant difference in biochemical pregnancy rates (p = 0.022). The live birth rate was also higher in the compaction group (21.6%) compared to the non-compaction group (7.7%), showing a borderline statistical difference in the Chi-square analysis (p = 0.072). No significant difference in serum progesterone levels was observed between patients with and without endometrial compaction. Limitations: While simultaneous evaluation of endometrial compaction and serum progesterone with adjustment for confounders enhanced internal validity, the study’s single-center design and limited sample size restricted subgroup analyses and statistical power. Further research with larger cohorts and consideration of progesterone administration routes is needed to confirm and extend these findings. Conclusion: Endometrial compaction on the day of embryo transfer is linked to increased rates of biochemical and clinical pregnancies. This association remains consistent regardless of serum progesterone levels, highlighting the independent influence of endometrial compaction on pregnancy success.