Selection of embryos for transfer in IVF: ranking embryos based on their implantation potential using morphological scoring.

The selection of embryos based on morphology is still the core of daily laboratory practice in IVF/intracytoplasmic sperm injection. At present, the selection of embryos is primarily based on experience and local protocols. Since an evidence-based ranking strategy for embryos on day 3 is currently lacking, this work constructed a multivariable prediction model to rank embryos according to their implantation potential. A total of 6021 fresh embryo transfers between January 2004 and July 2009 were included, eight potential predictive factors were evaluated and a prediction model was developed using multivariable logistic regression. The model was externally validated with data from couples treated between August 2009 and September 2011 in the same clinic. Five factors were included in the final prediction model: early cleavage, number of blastomeres on days 2 and 3 and morphological score and presence of morula on day 3. With validation, the model showed moderate discriminative capacity (c-statistic 0.70) and calibrated well and was able to distinguish embryos with high ongoing implantation potential from embryos with moderate or low ongoing implantation potential. The model can be used by embryologists as an objective tool to rank embryos according to implantation potential, thereby aiding the selection of embryos for transfer.