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Both progesterone and mifepristone treatment increased average fetal weights. However, progesterone treatment did not affect the number of large or small fetuses or placentas. In contrast, mifepristone specifically reduced the number of small fetuses and placentas but had no effect on the number of large fetuses or placentas. These results suggest that progesterone likely acted by accelerating fetal growth. The faster growth rate may have increased the risk of loss due to limitations in uterine capacity. Mifepristone treatment, by contrast, may have decreased the survival rate of smaller fetuses in some way, causing them to be underrepresented in the litter and thus decreasing litter size. Decreased progesterone during early pregnancy might influence many aspects of either uterine or conceptus physiology, resulting in the preferential loss of smaller conceptuses. Changes in uterine protein secretion, uterine blood flow, conceptus elongation, placental development, or aspects of fetal function are all possible mechanisms. A high plane of nutrition has been reported to impair embryonic survival, and this effect may be mediated by low progesterone concentration during the first few days of pregnancy. Progesterone treatment during early pregnancy has been suggested as a way to alleviate this effect, improve embryonic survival, and thus improve litter size.