The assessment of oocytes showing only one pronucleus during assisted reproduction is associated with uncertainty. A compilation\nof data on the genetic constitution of different developmental stages shows that affected oocytes are able to develop into haploid,\ndiploid, and mosaic embryos with more or less complex chromosomal compositions. In the majority of cases (?80%), haploidy\nappears to be caused by gynogenesis, whereas parthenogenesis or androgenesis is less common. Most of the diploid embryos\nresult from a fertilization event involving asynchronous formation of the two pronuclei or pronuclear fusion at a very early stage.\nUniparental diploidy may sometimes occur if one pronucleus fails to develop and the other pronucleus already contains a diploid\ngenome or alternatively a haploid genome undergoes endoreduplication. In general, the chance of obtaining a biparental diploid\nembryo appears higher after conventional in vitro fertilization than after intracytoplasmic sperminjection. If a transfer of embryos\nobtained from monopronuclear oocytes is envisaged, it should be tried to culture them up to the blastocyst since most haploid\nembryos are not able to reach this stage. Comprehensive counselling
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