The relationship and optimal threshold of endometrial thickness with early clinical pregnancy in frozen embryo transfer cycles.

OBJECTIVE

To investigate the associations of endometrial thickness with pregnancy outcome in frozen embryo transfer (FET) cycles.

METHODS

A retrospective cohort study was performed looking at 1627 FET cycles from the Reproductive Medicine Center of the study hospital between January 2017 and July 2018. Endometrial ultrasonographic characteristics were recorded on the embryo transfer day in FET cycles.

RESULTS

A total of 1627 FET cycles were included. The endometrial thickness was independently associated with clinical pregnancy outcomes after adjusting for potential confounders (odds ratio 1.06; 95% confidence interval [CI] 1.01-1.12). A non-linear relationship was detected between endometrial thickness and pregnancy outcomes, whose point was 10.9 mm. The effect size of the left and right sides of the inflection point were 1.16 (95% CI 1.07-1.25) and 0.89 (95% CI 0.78-1.01), respectively. Subgroup analysis showed that the correlation between endometrial thickness and pregnancy outcome was consistent in all subgroups.

CONCLUSION

The relationship between endometrial thickness and pregnancy outcome was non-linear and there is an inflection point. When endometrial thickness was less than 9.5 mm, it was positively related to clinical pregnancy rate. If it was beyond the inflection point, the pregnancy rate does not increase significantly.