Uterine capacity in the pig reflects a combination of uterine environment and conceptus genotype effects

Prenatal losses in U.S. pig breeds range from 30 to 50%, of which greater than 75% occurs before d 30 of gestation and is thought to result from littermate asynchrony. Numbers of embryos can be experimentally increased to d 30 using superovulation (excess ova shed) and superinduction (transfer of embryos to an already pregnant uterus); however, these females farrow the same number of pigs as untreated controls. These data demonstrate that between d 30 and parturition there are significant additional periods of conceptus loss, which has led to the conclusion that uterine capacity (i.e., the number of conceptuses a sow uterus can accommodate) is the major limitation to litter size in the pig. The special importance of uterine capacity in the pig may result from the noninvasive epitheliochorial type of placentation in this species, making the surface area of attachment between the placenta and endometrium a limiting factor. In devising selection schemes for this trait, one could logically conclude the