Optimum conceptus (fetal and placental) growth appears to be achieved through the combination of both uterine and fetal-directed mechanisms. The uterus, in which a litter of conceptuses develops, has the major impact on limiting the size each conceptus ultimately attains; however, the genotype of each conceptus can differentially resist this effect, resulting in littermate conceptuses of different sizes. In contrast, the genotype of the conceptus modulates placental vascular density and thus placental efficiency. The ability to both limit placental size while at the same time increase placental vascular density of conceptuses of U.S. pig breeds would allow pork producers the potential of achieving an optimal litter size and increasing profitability.