Serum levels of progesterone, 5 alpha-dihydroprogesterone and hydroxy-5 alpha-pregnanones in the prepartum and postpartum equine.

Blood samples were collected from eight mares for ten days before and two days following parturition. Progesterone, 5 alpha-dihydroprogesterone and hydroxy-5 alpha-pregnanones were extracted from serum, separated by Sephadex LH-20 column chromatography and quantitated by radioimmunoassay. Progesterone levels ranged from 8.5 to 4.1 ng/ml among mares with normal parturition and decreased to 1.2 ng/ml after parturition. One mare with a retained placenta maintained the prepartal levels of progesterone into the postpartum period. 5 alpha-dihydroprogesterone increased to a level of 133 ng/ml at 80 hrs before parturition and began decreasing and reached a level of 9.2 ng/ml in the postpartum period. Levels of hydroxy-5 alpha-pregnanones were variable with a high of 1690 ng/ml and declined to a postpartum level of 60 ng/ml. The timing of the decline varied among the mares. The possible source and significance of these progestins remain speculative.