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The slower rate of conceptus development in Meishan gilts has been suggested to be due in part to decreased conceptus estrogen secretion in this breed. However, others have suggested that both the decreased estrogen secretion and the slower growth rate are a consequence of the decreased uterine protein secretion of the Meishan. In support of the hypothesis that the rate of conceptus development in the Meishan is due to conceptus estrogen secretion, estrogen treatment of Meishan gilts on Days 12 and 13 of pregnancy significantly increased placental weights. This increase in placental weights was not associated with a decrease in litter size. However, the number of observations in this experiment may have precluded obtaining a significant association between the two traits. It remains uncertain what effect estrogen treatment might have on placental weights and uterine capacity in European breed gilts. Mifepristone (also known as RU486) has been reported to be a progesterone and glucocorticoid receptor antagonist. This activity forms the basis of the use of mifepristone as a contraceptive. A single dose of mifepristone of sufficient magnitude abrogates pregnancy in humans. However, its effects on pregnancy in swine have not been studied. The ability of mifepristone to interfere with the interaction of progesterone and its receptor offered the possibility that mifepristone might be used to delay the normal response of the uterus to progesterone during pregnancy. We predicted that this delay in the onset of progesterone effects on the uterus would delay both uterine protein secretion and conceptus development, given that early progesterone treatment accelerated both traits. Such an effect would be similar to that in the Meishan breed, suggesting that mifepristone might be useful in improving uterine capacity and litter size.