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Previous studies have demonstrated that both protein secretion and conceptus development are highly variable and change rapidly at this stage of pregnancy (e.g., conceptuses change from spherical to filamentous blastocysts within 24 h). A factor contributing to this variation may be the use of once daily estrous detection in this experiment; the interval from first detection of estrus to ovulation in gilts varies from 23 to 48 h. By chance, the gilts treated with 100 mg mifepristone may have been more advanced than the control gilts, resulting in three of the nine gilts already having filamentous blastocysts by Day 11. However, the statistical analysis indicated a P value of 0.07 for conceptus diameter and of <0.05 for acid phosphatase, suggesting that it is more likely that this stimulation is a real effect of a low dose of mifepristone. Many physiological explanations are possible, including potential effects on oviduct physiology, but without further information we can only speculate on the cause of the acceleration in conceptus development and protein secretion. Nevertheless, the 400-mg dose decelerated conceptus development and uterine protein secretion as expected, allowing a test of this treatment on uterine capacity.