Embryonic mortality and the uterine environment

Embryonic mortality in early pregnancy is a frequent occurrence in most mammalian species. It continues to have a major impact on the livestock industry: in cattle, for example, about 35% of all conceptuses die before the third month of gestation. In the human, there is evidence that a high proportion of pregnancy losses are associated with chromosomal anomalies (Simpson, 1980; Angell, Hillier, West et al. 1988). In ruminants, however, chromosomal analysis of early embryos (Berepubo & Long, 1983) coupled with the results of numerous embryo transfer experiments (e.g. Markette, Seidel & Elsden, 1985) indicate that the maternal environment in which the embryo develops is of greater importance. Many studies have been aimed at understanding what aspects of this 'maternal environment' are crucial, and how these are regulated. This commentary will summarize the information which has been gained historically and will evaluate the additional inputs which are now coming from the