Does Endometrial Thickness or Compaction Impact the Success of Frozen Embryo Transfer? A Cohort Study Analysis

<b>Background</b>: In frozen embryo transfer (FET) cycles, optimal endometrial thickness on ovulation day is typically 7-8 mm before progesterone administration. Recent studies have highlighted the significant inverse correlation between ongoing pregnancy rates and changes in endometrial thickness during the secretory phase after progesterone exposure, particularly by the day of embryo transfer (ET). This study aims to investigate how changes in endometrial thickness from the end of the proliferative phase to ET impact FET outcomes. <b>Methods</b>: We conducted a prospective observational cohort study involving 247 FET cycles, divided into hormonally based (102) and ovulatory-based (145) groups. Patients were monitored through transvaginal ultrasound (TVS) and blood tests. On ET day, we assessed endometrial thickness and changes compared to the last day of the proliferative phase, defining endometrial compaction as a decrease in thickness. We analyzed data to identify factors predicting cycle outcomes. <b>Results</b>: The study reported chemical and clinical pregnancy rates of 47.4% and 38.1%, respectively. Endometrial compaction was observed in 37.2% (92/247) of cycles, with corresponding rates of 48.9%, 32.6%, and 29.5% for chemical, clinical, and ongoing pregnancies, compared to 46.4%, 41.3%, and 28.9% in cycles without compaction. These differences were not statistically significant, and patterns of endometrial thickness change were similar across different FET protocols and progesterone exposure durations. <b>Conclusions</b>: The main factors influencing cycle outcomes were maternal age, embryo transfer protocol, and endometrial thickness on ET day, with endometrial compaction showing no correlation with improved outcomes.