Abstract STUDY QUESTION How does nucleus status at the two-cell stage predict blastocysts formation and clinical outcome after single blastocyst transfer? SUMMARY ANSWER Binucleated embryos at the two-cell stage (2BI) show higher rates of good quality blastocyst formation, pregnancy and live birth compared to those with one nucleus in each blastomere (2MONO), whereas true multinucleated embryos at the two-cell stage (2MULTI) show lower rates of good quality blastocyst formation and pregnancy compared to 2MONO embryos. WHAT IS KNOWN ALREADY The introduction of time-lapse culture has made it possible to study nucleus status at the two-cell stage more consistently and it shows that multinucleation at the two-cell stage (2MN) is a common event. The effect of 2MN is still unclear. High numbers of 2MN with the potential to develop to blastocysts that become clinical pregnancies and result in birth of healthy babies with no impaired perinatal outcome have been reported. However, some studies have found 2MN to be associated with impaired implantation and live birth. Furthermore, knowledge on how the different subgroups of multinucleation affects the IVF outcome is limited. STUDY DESIGN, SIZE, DURATION A non-interventional retrospective study was performed in a public fertility clinic. Blastocyst formation data from 223 women attending their first IVF cycle between May 2016 and December 2018, and clinical outcome data from 1314 single blastocyst transfers between May 2014 and December 2018 were used for the study. Fresh and frozen-thawed embryo transfers were included. PARTICIPANTS/MATERIALS, SETTING, METHODS Embryos were cultured until the blastocyst stage in a time-lapse incubator and nucleus status at the two-cell stage, the Gardner score and other morphokinetic parameters were annotated. We compared blastocyst development and clinical outcome, including positive hCG, ongoing pregnancy and live birth, of embryos with 2BI and/or 2MULTI blastomeres to 2MONO embryos. MAIN RESULTS AND THE ROLE OF CHANCE Embryos with 2BI in one blastomere (2BI1) were twice as likely to develop to good quality blastocysts (odds ratio (OR) 2.54, 95% CI 1.30–4.95, P = 0.006) compared to 2MONO embryos. Embryos with 2MULTI in both blastomeres (2MULTI2) were significantly less able to develop to good quality blastocysts (OR 0.38, 95% CI 0.23–0.63, P < 0.001) compared to 2MONO embryos. Embryos with 2BI in both blastomeres (2BI2) had a significantly better chance of resulting in a positive hCG (OR 2.40, 95% CI 1.11–5.20, P = 0.027), ongoing pregnancy (OR 2.79, 95% CI 1.29–6.04, P = 0.009) and live birth (OR 3.16, 95% CI 1.43–6.95, P = 0.004) compared to 2MONO blastocysts after single blastocyst transfer. In contrast, 2MULTI2 embryos were significantly less likely to result in a positive hCG (OR 0.58, 95% CI 0.35–0.97, P = 0.036) and ongoing pregnancy (OR 0.51, 95% CI 0.28–0.94, P = 0.030) compared to 2MONO blastocysts. LIMITATIONS, REASONS FOR CAUTION Discrepancies among the existing studies regarding the definition of multinucleation may lead to different conclusions. Even though the distinction between binucleation and true multinucleation was a strength in our study design, a further distinction between true multinucleated and micronucleated embryos could be interesting to investigate in future studies. Also, we included any anucleated embryos in the 2MONO group. For the study of clinical outcomes, the patients were allowed to be included with more than one transfer cycle. Both fresh and thawed transfers were included. WIDER IMPLICATIONS OF THE FINDINGS We find it important to discriminate between binucleation and true multinucleation when evaluating embryo nucleus status at the two-cell stage. Embryos displaying 2BI1 and 2BI2 have significantly better good quality blastocyst formation rates and clinical outcome after single blastocyst transfers, respectively. 2MULTI2 embryos have impaired blastocyst development potential and poorer clinical outcomes. STUDY FUNDING/COMPETING INTEREST(S) H.S.N. received an unrestricted grant from Merck for 3 months’ normal salary for a medical Doctor (A.L.T.) to write the manuscript. Merck was not involved in the study design, analysis, interpretation of data, writing the paper or the decision to submit the manuscript for publication. H.S.N. has received speaker’s fees from Ferring Pharmaceuticals, Merck Denmark A/S, Astra Zeneca, Cook Medical and Ibsa Nordic (outside the submitted work). N.l.C.F. has received a grant from Gedeon Richter (outside the submitted work). The other authors did not report any potential conflicts of interest. All authors declared no conflicts of interest regarding this work. TRIAL REGISTRATION NUMBER N/A.