The procedure of in vitro fertilization, including ova collection and embryo culture and transfer, raises concern about the risk of abnormalities arising from the technique. In vitro versus in vivo comparison might be made in the study of preimplantation embryos and spontaneous abortions, the results of amniocentesis, and the findings among fetal deaths and live births. In vivo, 40% to 50% of implantation blastocysts are estimated to have a chromosomal abnormality, over 99% of which are estimated to be eliminated during the course of pregnancy. In principle, the use of early spontaneous abortions can be more efficient than amniocentesis for detecting an increased risk of chromosomal abnormalities at birth. Unless in vitro fertilization in humans strongly contradicts the experience in domestic animal reproduction, which suggests no increased risk of abnormalities at birth, a large number of births would be required to provide a definitive assessment of risk.