Monozygotic twins (MZT) are 2.5 times more frequent in ART than in natural conceptions. A number of ART-related mechanisms have been probably linked with MZT. Studies that retrospectively analyze the time-lapse (TL) records resulting in MZT suggest that some morphokinetic traits of the inner cell mass and the trophectoderm could be predictors of MZT, but results are controversial. We present the complete TL record of one case of MZT that split itself at the very moment of the division into two cells, with one of the cells coming out through a hole in the zona pellucida (ZP). Both resulting embryos developed normally, and were vitrified. It is suggested that the hole in the ZP may facilitate the extrusion of some cells of the <day 4 embryo and that this cell development is not constrained by being inside the ZP. Despite the lack of the inhibition of the ZP itself or the influence of the other embryo cells, the totipotent cell was then able to develop correctly from the start. Moreover, the embryo inside the ZP compensated for the loss of this cell apparently without problems. Our findings are discussed in the context of previous literature and ethical problems are addressed.
Keywords: Blastocyst; cleavage–stage embryo; embryo splitting; embryonic development; monozygotic twins; time-lapse recording.