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These mechanisms appear to fail below a fetal weight of 800 g. Although numerous other factors also contribute to the loss of small piglets, improper brain development that occurs in these small fetuses may contribute to perinatal and neonatal losses. These data indicate that early progesterone treatment decreased uterine capacity at 105 days of gestation in UHO gilts, possibly by accelerating fetal growth. Early progesterone treatment had limited effects on litter size and birth weights in intact white crossbred gilts. Mifepristone treatment on Day 2 of gestation decreased both conceptus diameter and uterine protein secretion measured on Day 11. Surprisingly, mifepristone treatment on Day 2 also decreased uterine capacity measured on Day 105. The decrease in uterine capacity in response to mifepristone treatment apparently resulted from a decrease in the number of small (less than the control mean for fetal weight) fetuses at 105 days of gestation; the number of large fetuses was unaffected. These results suggest that both too much and too little progesterone during the first 2 or 3 days of pregnancy have a detrimental effect on uterine capacity. Efforts to define the optimum naturally occurring rate of progesterone rise combined with genetic selection to optimize this trait in pig populations could lead to increased uterine capacity and litter size.