Enhancing predictions of embryo implantation in thin endometrium: a comparative analysis of endometrial thickness in frozen versus fresh IVF cycles

This study investigated whether the difference between endometrial thickness in the frozen embryo transfer (FET) cycle and the preceding fresh stimulated cycle predicts clinical pregnancy in women with thin endometrium. A retrospective analysis of 335 cycles in 227 women (age < 43) with endometrium < 8 mm in the FET cycle was conducted. Endometrial thickness was assessed using the ratio (ET^frozen/fresh) and absolute difference (ET^fresh-frozen) between the maximal endometrial thickness measured during the stimulated IVF cycle and that measured in the FET cycle. We hypothesized that a smaller difference and higher ratio during FET will be associated with higher pregnancy rate. No embryo implantation occurred in women with an FET endometrial thickness < 5.9 mm and an ET^frozen/fresh < 0.86. Cycles with a positive beta-human chorionic gonadotropin (ß-hCG) showed a significantly lower mean ET^fresh-frozen (1.89 ± 1.78 vs 2.30 ± 1.81, p = 0.03), and a significantly higher ET^frozen/fresh ratio (0.82 ± 0.17 vs. 0.77 ± 0.15, p = 0.024). Endometrial thickness in the FET cycle normalized to the preceding stimulated cycle is associated with implantation and may constitute a novel predictor of FET success in women with thin endometrium.