To Err is Human: The Complex Nature of Human Reproduction and Prenatal Development

Developmental errors occur frequently early in gestation and at least 10% of human embryos are morphologically and/or cytogenetically abnormal around the fifth week of gestation. The causes of developmental anomalies are genetic, environmental, or a combination of multiple factors. It appears that developmental errors and reproductive losses occur exceptionally frequently in the human species, but most abnormal embryos/fetuses die in utero and end in spontaneous abortion. High mutation rates in humans can be deleterious for human health and survival, but mechanisms may exist that favor normal over faulty conceptions and bring about natural elimination of abnormal human conceptuses.