[The value of blood progesterone determination about 18 days post ovulation for pregnancy testing in mares]

The reliability of determination of the plasma progesterone level within approximately eighteen days after ovulation in the pregnancy diagnosis of mares is examined in the present study. Studies were done in seventy-five mares, a number of which were served or inseminated during several cycles so that a total number of eighty-seven blood samples were obtained. On the analogy of other authors, the progesterone level above which mares were believed to be pregnant and below which they were assumed to be non-pregnant, was set at 2 ng/ml. The twenty-five mares in which the level was below 2 ng/ml. were indeed found to be non-pregnant at that time of examination, the levels of six samples not being determined in view of the predictable results. Of the sixty mares showing progesterone levels of more than 2 ng/ml., forty-seven were ultimately found te be pregnant, fifty-five had probably been pregnant on day 18 and five were non-pregnant and had not been pregnant. The reliability of a diagnosis negative for pregnancy, based on determination of the progesterone level, was one hundred per cent in these cases, that of a diagnosis positive for pregnancy being 78.3 per cent. When early embryonal mortality was taken into account, reliability of a diagnosis positive for pregnancy was 91.6 per cent. Overall reliability was 84.7 per cent.