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In the Meishan swine breed, which has greater fertility than European breeds, uterine protein secretion and conceptus development and conceptus estrogen secretion during early pregnancy are reduced compared with European breeds. One hypothesis is that this slower conceptus development during early pregnancy results in smaller placentas during later pregnancy and allows the uterus to accommodate more fetuses, thus increasing uterine capacity, which is defined as the number of live fetuses that can be maintained by the uterus during gestation. Previous experiments also indicated that treatment of pregnant white crossbred gilts with progesterone on Days 2 and 3 of pregnancy accelerated the normal changes in uterine protein secretion occurring at the time of maternal recognition of pregnancy (Day 11). Conceptus estrogen secretion also was increased on Day 11 after early progesterone treatment, which suggested that conceptus development was accelerated. Thus, because early progesterone treatment had effects opposite to those present in the Meishan breed, we hypothesized that this treatment may increase the size of the conceptus during later gestation and decrease uterine capacity and litter size.