The assessment of oocytes showing only one pronucleus during assisted reproduction is associated with uncertainty. A compilation of data on the genetic constitution of different developmental stages shows that affected oocytes are able to develop into haploid, diploid, and mosaic embryos with more or less complex chromosomal compositions. In the majority of cases (~80%), haploidy appears to be caused by gynogenesis, whereas parthenogenesis or androgenesis is less common. Most of the diploid embryos result from a fertilization event involving asynchronous formation of the two pronuclei or pronuclear fusion at a very early stage. Uniparental diploidy may sometimes occur if one pronucleus fails to develop and the other pronucleus already contains a diploid genome or alternatively a haploid genome undergoes endoreduplication. In general, the chance of obtaining a biparental diploid embryo appears higher after conventional in vitro fertilization than after intracytoplasmic sperm injection. If a transfer of embryos obtained from monopronuclear oocytes is envisaged, it should be tried to culture them up to the blastocyst since most haploid embryos are not able to reach this stage. Comprehensive counselling of patients on potential risks is advisable before transfer and a preimplantation genetic diagnosis could be offered if available. 1. Introduction The technology of assisted reproduction aims at achieving oocyte fertilization by incubation of cumulus-intact oocytes in the presence of a defined number of motile spermatozoa (conventional in vitro fertilization, IVF) or by injection of single spermatozoa into denuded, cumulus-free oocytes (intracytoplasmic sperm injection, ICSI). Both procedures are followed about 16 to 20 hours later by the so-called pronucleus check. Here, successful and normal fertilization is identified by the appearance of two pronuclei (PN) in the ooplasm and detection of two polar bodies in the perivitelline space, whereas the presence of more than two PN is considered to be associated with genetic disorders, mostly triploidy [1]. Consequently, these multipronuclear oocytes are excluded from further cell culture and embryo transfer. In contrast, recommendations on the treatment of oocytes displaying only one pronucleus are accompanied by greater uncertainty. In case of parthenogenetic activation, one should expect the formation of a haploid embryo with exclusively maternal chromosomes and therefore transfer should be cancelled. If, however, the PN had appeared asynchronously or underwent an undetected fusion, diploid biparental and
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