The number of pigs weaned per sow per year has increased substantially in recent years due mainly to changes in husbandry with probably some contribution from breed substitution and crossbreeding (Haley et al 1986). Because of emerging difficulties in relation to responses to continued selection for leaness and reassessment of the economic importance of prolificacy in pigs there is considerable interest in selection for increased litter size. It is generally agreed that the heritability of litter size is about 0.1 and attempts to increase litter size by within population selection have not been particularly successful. On the other hand selection for increased ovulation rate in pigs has been effective but the correlated change in litter size has been small. This apparent failure to achieve significant correlated response in litter size has been attributed to a negative genetic association between ovulation rate and embryo survival. However, there is very little information on the magnitude of genetic variation in embryo survival or its relationship with ovulation rate.