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BACKGROUND: In order to select the best blastocyst for transfer, in humans, three morphological parameters have routinely been used, i.e. degree of blastocoele expansion and appearance of both the trophectoderm (TE) and the inner cell mass (ICM). Although it has been shown that blastocysts with highest scores for all three parameters achieve highest implantation rates, their independent ability to predict pregnancy outcome remains unclear. METHOD: This study is a retrospective analysis of 1117 fresh day 5 single blastocyst transfers and their live birth outcome related to each morphological parameter. RESULTS: All three parameters had a significant effect on live birth however, once adjusted for known significant confounders, it was shown that TE was the only statistically significant independent predictor of live birth outcome. CONCLUSIONS: This study has shown, for the first time, the predictive strength of TE grade over ICM for selecting the best blastocyst for embryo replacement. It may be that, even though ICM is important, a strong TE layer is essential at this stage of embryo development, allowing successful hatching and implantation.
Ahlström et al. (Mon,) studied this question.