Endometrial compaction is associated with increased clinical and ongoing pregnancy rates in unstimulated natural cycle frozen embryo transfers: a prospective cohort study

Purpose To investigate the association between endometrial compaction and pregnancy rates in unstimulated natural cycle frozen embryo transfers. Design A single-center prospective cohort study. Endometrial thickness by transvaginal ultrasound and blood progesterone levels on the day of ovulation and the day of embryo transfer were evaluated in patients undergoing natural cycle frozen embryo transfer. Compaction was defined as > 5% decrease in endometrial thickness between ovulation day and day of transfer. Clinical and ongoing pregnancy rates in cycles with and without compaction were compared. Results Seventy-one women were included, of which 44% had endometrial compaction, with similar rates when subdividing the patients by day of transfer (day 3 or day 5). Clinical and ongoing pregnancy rates were higher in the compaction group compared to the non-compaction group (0.58 vs. 0.16, P < 0.001; 0.52 vs. 0.13, P < 0.001 respectively). Subdividing by degree of compaction > 10% and > 15% revealed similar pregnancy rates as > 5%, with no added benefit to higher degrees of compaction. Conclusions About half the patients in our study undergoing unstimulated natural cycle frozen embryo transfer experienced compaction of the endometrium, occurring as early as day 3 post-ovulation. This was significantly correlated with increased clinical and ongoing pregnancy rates.