Objectives: To investigate the variation of endometrial responsiveness between cycles within the same women undergoing assisted reproduction.
Methods: The sonographic endometrial thickness in ovarian stimulation cycles was compared with that of subsequent natural cycles. One hundred and thirty-six ovarian stimulation cycles of in-vitro fertilization and embryo transfer were evaluated. Women who did not conceive in in-vitro fertilization cycles were subsequently seen in natural cycles (n = 97) or the next in-vitro fertilization cycle (n = 39). Based on a receiver-operating characteristics (ROC) curve using endometrial thickness to predict pregnancy, the first in-vitro fertilization cycles were classified according to the endometrial thickness as optimal (> 8 mm) in 98 cycles, or suboptimal (< or = 8 mm) in 29 cycles. Similarly, spontaneous cycles were classified as suboptimal (< or = 7 mm) in 28 cycles and optimal (> 7 mm) in 69 cycles.
Results: The pregnancy rates were significantly lower (P < 0.05; Fisher's Exact test) in the suboptimal group in both the in-vitro fertilization and frozen embryo transfer cycles. There was a strong correlation (r2 = 0.745) and a significant difference (P < 0.001; Wilcoxon signed rank sum test) between the endometrial thickness of stimulation and natural cycles.
Conclusion: It is possible to predict the occurrence of optimal or suboptimal endometrial response in natural cycles of women, after evaluation in stimulated cycles, with a high degree of reliability. Risk of implantation failure can be identified before subsequent treatment cycles and adjuvant therapeutic strategies may be planned to improve the endometrial response before embryo transfer.