Relationship of progesterone to early pregnancy loss in mares

Summary Exogenous progestins are often given to pregnant mares but many questions remain unanswered as to the relationship between concentrations of progesterone and embryo survival. Thus, embryos were collected 7 days after ovulation and transferred into either ovarian-intact recipients (I,n=15) or ovariectomized recipients which were given 100 mg progesterone (L,n=15) or 1,500 mg progesterone (H,n=15) daily. Concentrations of progesterone were determined in all mares from days 0 to 13. Recipients were examined for pregnancy with ultrasonography during days 11 through 25. Between days 28 and 100, twelve ovarian-intact, pregnant recipients were monitored for follicular and luteal changes and bled for determination of progesterone concentrations for 4 days at 2-week intervals. Twelve ovariectomized pregnant recipients were taken off injectable progesterone days 25-30 and pregnancy maintained to day 100 with an oral progestin, altrenogest. Concentrations of progesterone in these mares were determined every 3 days. Progesterone treatment had no effect on maintenance of pregnancy to day 100 or size of embryonic vesicle (P>0.05). Levels of progesterone during days 1 to 13 were 1.3 to 3.0 ng/ml, >25 ng/ml and 7 to 9 ng/ml for groups L, H and I, respectively. Concentrations of endogenous progesterone in ovariectomized pregnant mares became >1 ng/ml by day 91 of gestation indicating placental progesterone secretion. Increased concentrations of progesterone from days 60 to 100, seen in ovarian-intact, pregnant mares, were primarily due to secretions from corpora lutea. There was, in fact, a correlation between the number of 2°CL and progesterone level (r2=0.74). Initial formation of secondary CL's (2°CL) occurred from 28 to 51 days, with peak formation of 2°CL on days 55 to 70 of gestation.