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Results of experiment 2 indicated a slight negative effect on overall pregnancy rate. However, for those gilts in which pregnancy was established, progesterone did not affect litter size, arguing against these mechanisms as an explanation for the results of experiment 1. In addition, results of experiment 3 indicate that the number of blastocysts was not reduced by Day 11 in gilts given doses of mifepristone up to 400 mg, suggesting that fertilization rate and oviductal transport were unaffected in mifepristone-treated gilts. Interference by mifepristone with the process of elongation, which occurs after Day 11, is still a possibility. However, decreased uterine capacity should have been reflected in an increased incidence of mummies, which would not occur if the explanation for the decrease in litter size was either failure of fertilization or decreased early embryonic survival due to failure or interference with elongation. Although the difference was not significant, the number of mummies was higher in both the progesterone- and the mifepristone-treated gilts compared with control gilts, consistent with at least a portion of the differences in litter size between control gilts and progesterone- or mifepristone-treated gilts being due to decreased uterine capacity. It is currently unclear whether all fetuses that die after Day 30 of gestation (those lost due to uterine capacity) remain present in the uterus as mummies for the duration of gestation.