• Laila Noli, Yaser Dajani, Antonio Capalbo, Jean Bvumbe, Laura Rienzi, Filippo Maria Ubaldi, Caroline Ogilvie, Yacoub Khalaf, and Dusko Ilic.
• Division of Women's Health, Faculty of Life Sciences and Medicine, King's College London and Assisted Conception Unit, Guys' Hospital, London SE1 9RT, UK.
• Hum. Reprod. 2015 Dec 1; 30 (12): 2774-84.

Study QuestionIs the quality of the human embryos generated by twinning in vitro comparable to the quality of the embryos created by fertilization?Summary AnswerOur data suggest that the human twin embryos created in vitro are unsuitable not only for clinical use but also for research purposes.What Is Known AlreadyPregnancy from in vitro generated monozygotic twins by embryo splitting or twinning leads to live birth of healthy animals. Similar strategies, however, have been less successful in primates. Recent reports suggest that the splitting of human embryos might result in viable, morphologically adequate blastocysts, although the qualitative analyses of the embryos created in such a way have been very limited.Study Design, Size, DurationThis study was a comparative analysis of embryos generated by twinning in vitro and the embryos created by in vitro fertilization.Participants/Materials, Setting, MethodsWe analysed morphokinetics and developmental competence of 176 twin embryos created by splitting of 88 human embryos from either early (2-5 blastomeres, n = 43) or late (6-10 blastomeres, n = 45) cleavage stages. We compared the data with morphometrics of embryos created by in vitro fertilization and resulting in pregnancy and live birth upon single blastocyst transfer (n = 42).Main Results And The Role Of ChanceThe morphokinetic data suggested that the human preimplantation development is subjected to a strict temporal control. Due to a 'developmental clock', the size of twin embryos was proportionate to the number of cells used for their creation. Furthermore, the first fate decision was somewhat delayed; the inner cell mass (ICM) became distinguishable later in the twin than in the normal blastocysts obtained through fertilization. If an ICM was present at all, it was small and of poor quality. The majority of the cells in the twin embryos expressed ICM and trophectoderm markers simultaneously.Limitations, Reasons For CautionWe created monozygotic twins by blastomere separation from cleavage stage embryos. Embryo twinning by blastocyst bisection may circumvent limitations set by the developmental clock.Wider Implications Of The FindingsTaken together, our data suggest that the human twin embryos created in vitro are unsuitable not only for clinical use but also for research purposes.© The Author 2015. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

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