Several inherited diseases can now be diagnosed by genetic analysis of single cells biopsied from human eggs and preimplantation embryos following in vitro fertilization (IVF). 'At risk' couples can, therefore, have only unaffected embryos replaced in the uterus and avoid the possibility of terminating a pregnancy that might only be diagnosed as affected later is gestation. Single-cell genetic analysis has also provided powerful tools for studying genetic defects arising during early human development. Recent studies of cleavage-stage human embryos have revealed an unexpectedly high incidence of postzygotic chromosomal abnormalities, which might arise because of a lack of cell-cycle checkpoints before the embryonic genome is activated. These genetic abnormalities are likely to contribute to early pregnancy loss and have important implications for improving pregnancy rates in infertile couples by assisted reproduction.