Substantial prenatal mortality has been observed in all mammals studied, although there are significant differences between species in the extent and timing of the death. There are two reasons why it is important to define the causes of such loss. First, it is of fundamental interest to understand why prenatal loss continues to occur despite natural selection for efficient reproduction. Second, there may be practical applications arising from such knowledge if it can be used to increase the survival of embryos. Great distress is caused to those people who suffer infertility, particularly if it is a result of repeated miscarriage. Furthermore, a substantial economic loss follows prenatal death in farm animals. This death leads to a reduction in litter size in pigs and prolific sheep and, in cattle and sheep with only one ovulation at each oestrus, an increased interval between births.
The causes of prenatal mortality have been