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The first experiment demonstrated the effect of early progesterone treatment on uterine capacity in pigs. Uterine capacity, measured as the number of live fetuses in UHO gilts, was decreased in progesterone-treated gilts compared with control gilts. This decrease occurred in association with increased fetal weights at 105 days of gestation. Further analysis suggested that the increased fetal weight was not due to differences in survival of large or small fetuses in progesterone-treated gilts. Thus, early progesterone treatment may have accelerated fetal development. In contrast, in the second experiment, early progesterone treatment did not influence litter size, stillbirth rate, or birth weights in intact pregnant gilts but did significantly shorten gestation length by approximately 0.5 days. This result again suggests that early progesterone treatment accelerated the normal course of pregnancy, causing earlier parturition. This acceleration occurred in intact gilts, with no effect on birth weight and no lasting effect on piglet growth rate after birth. These results led us to hypothesize that mifepristone, a progesterone receptor antagonist, would be useful in delaying conceptus development and uterine protein secretion, creating a uterine environment that increases uterine capacity and be similar to the oviductal and uterine environment of the Meishan breed.