Humans are surprisingly infertile with young fertile women getting pregnant in only 30 % of the cycles after natural intercourse. Chromosomal abnormalities are a major cause of early pregnancy loss in humans. Recently, we developed a genome wide single blastomere screening method and revealed a remarkably high level of chromosomal instability in embryos following in vitro fertilization (IVF). In addition to whole chromosome anomalies also high numbers of chromosomal rearrangements were observed. Those chromosomal anomalies are hypothesized to underlie the low pregnancy rates both in vivo and in vitro. However, due to ethical concerns about human embryo research, this has not yet been proven. In cattle, early pregnancy loss is a major problem. Young fertile cows have a 70 % chance to conceive after insemination, but pregnancy drops to 40 % after transfer of in vitro roduced embryos. Some studies suggest that those reduced pregnancy rates could also be due to chromosomal instability and hence, that bovine embryogenesis mimics human development. Using bovine as a model, we want to study (1) early bovine embryos derived from the oviduct to determine whether chromosomal instability is not only an in vitro phenomenon but also occurs in vivo; (2) whether early cleavage kinetics of bovine embryos in vitro can foretell chromosomal aberrations; (3) if an embryo can get rid of chromosomal errors during further development in the uterus. (4) Furthermore, we will use IVF bovine embryos to test culture conditions which may influence chromosome aberrations during in vitro procedures.