The likely importance of chromosomal abnormalities as a cause of early pregnancy loss was made clear by stud­ies of spontaneous abortions in which more than 50% chromosome abnormalities were encountered [1,2] Since the introduction of in vitro fertilization in humans, the frequency of chromosome abnormalities in oocytes, zy­gotes and early stages of embryo development, have also been studied. This type of study is complicated by the fact that in most countries legal restrictions hamper the study of unselected series of zygotes and early embryos. There­fore, these types of human embryos for study originate within the context of treatment for infertility and do not reflect the normal situation [3]. Furthermore, in the early days, many stages of unsuccessful fertilization and devel­opment could not be studied at all. The introduction of new methods, that are not based on dividing cells but at the molecular detection of abnormalities in interphase cells, has allowed more unselected studies to be carried out. The various types of fluorescent in situ hybridization (FISH) and other molecular methods now available, have led to reliable estimates of aneuploidy rates.