Multinucleation in the Two-Cell Stage Embryo Matters

Abstract

Background: Blastomere multinucleation (MN), defined as the presence of more than one nucleus per cell, in the cleavage-stage embryo, is a well-documented phenomenon1. This nuclear anomaly has become more visible with the introduction of time-lapse incubation and an understanding of the potential outcomes of these embryos is necessary to inform decision-making and manage patient expectations. Aim: To determine rates of blastocyst utilisation and euploidy for embryos with MN at the two-cell stage (MN2) in comparison to mononucleated and binucleated embryos. Method: A retrospective analysis was performed at a single IVF unit, of embryos with known two-cell nucleation status from October 2019 to March 2021. Blastomere nucleation was assessed, via time-lapse monitoring (EmbryoScope; UnisenseFertiliTech), and categorised as mononucleate, binucleate or MN in all fertilised oocytes. Blastocyst utilisation rates (number of embryos frozen+transferred/number of oocytes fertilised) were compared together with euploidy rates in embryos that had preimplantation genetic testing for aneuploidy. Outcomes were modelled using multilevel logistic regression to allow for lack of independence between embryos when grouped by patient and cycle, and adjusted for patient age. Results: Of the 4388 embryos included in this study, 68% were mononucleated, 14% binucleated and 18% MN. The likelihood of embryos becoming usable blastocysts based on nucleation status at the two-cell stage for mononucleated, binucleated and MN were 63%, 55%(p<0.001) and 35%(p<0.001) respectively. The euploidy rates for mono-, bi- and MN were 45%, 38%(p=0.09) and 31%(p=0.004) respectively. Predictive modeling, with adjusting for age, showed that a mononucleate two-cell embryo has 44% likelihood of becoming a euploid blastocyst, binucleate 36% and MN 28%. Conclusion: There was a clear reduction in rate of blastocyst utilisation and euploidy in binucleate and a further reduction in MN two-cell stage embryos, in comparison to mononucleate embryos, which should preferentially be prioritised for transfer.