The origins of genetic variation between individual human oocytes and embryos: implications for infertility.

Human fertility is low in comparison with that seen in other well-studied mammals. The main reason for this state of affairs seems to be the frequent occurrence and persistence of chromosomal errors in the human conceptus. Evidence obtained over the past two decades shows that the exceptionally high incidence of chromosomal anomalies seen in human preimplantation embryos is the result of errors that may occur at various stages during gamete and embryo formation. In rare cases, an error may exist or arise in the premeiotic germ cells; much more commonly it may arise during the first or second meiotic division in the male or female. Highly efficient cell cycle checkpoints in the male ensure that the incidence of aneuploidy in mature sperm is low compared to that in the oocyte. Most 3-day-old embryos created by IVF are chromosomal mosaics, and this persists to a lesser degree to the blastocyst stage on day 5. While aneuploidy of meiotic origin is a major factor affecting the fertility of older women, embryos from most younger women will have predominantly post-zygotic mitotic errors. Couples experiencing RIF are particularly likely to produce highly abnormal (chaotic) embryos by post-zygotic mechanisms.