The observed cumulative delivery rates in this study do not allow us to take a position in favor of SBT or eSET. An improvement in blastocyst cryopreservation may change this attitude.

Routine early developmental parameters are widely used in IVF centres to evaluate embryo development and fresh single-blastocyst transfer currently seems superior to single-embryo transfer. Would early morphological parameters help to choose the single blastocyst to be transferred, thereby improving the chances of implantation and live birth rate? This prospective observational study analysed the individual outcomes of 2617 embryos from 511 IVF couples scheduled for a single-blastocyst transfer. Embryo and blastocyst scores were constructed. There was a clear relationship between the kinetics and morphology of blastocysts and further implantation and live birth rate. There was a limited predictive value of embryo score with regard to blastocyst development and growth kinetics. Implanted and non-implanted blastocysts showed similar embryo scores. Thus usual morphological parameters on days 1 and 2 seem to have no additional value in indicating the right blastocyst to transfer. Non-invasive approaches might be helpful to increase the chances of implantation in the future.