PIF levels, day 10- post AI vs. control and day-20 post AI vs. control. PIF levels were determined by ELISA at different days after AI in cows with documented calf delivery. Levels were compared with non-pregnant never inseminated animal. A stringent +3SD threshold (99.74%) was used vs controls. As expected, non-pregnant control PIF levels overlap with a small number of day-10 post-AI subjects, whereas by day-20, there is clear stratification.