Treatment with human chorionic gonadotrophin before ovulation increases progestin concentration in early equine pregnancies.

Abstract For prevention of early conceptus loss in the horse, treatment with progestins has become common practice. In cattle, treatment with human chorionic gonadotrophin (hCG) during the early postovulatory phase stimulates endogenous progesterone synthesis, which is an important factor for maintenance of early pregnancy via stimulation of endometrial function and conceptus development. In the present study we have therefore investigated the influence of treatment with hCG either for induction of ovulation or during the early luteal phase on plasma progestin concentrations, size of the corpus luteum and size of the conceptus in early pregnant mares. We hypothesized that hCG treatment stimulates progestin secretion and conceptus development. In Experiment 1, induction of ovulation with hCG (1500IU i.v.; n =14) significantly increased progestin concentration between days 5 and 15 after ovulation compared to untreated controls ( n =28; p p n =12) did neither affect progestin secretion (e.g. day 8 hCG: 15.4±1.6, control: 17.6±1.2ng/ml) nor luteal tissue area (e.g. day 8 hCG: 9.0±0.7, control: 7.6±1.4cm 2 ) compared to untreated mares ( n =9). In conclusion, treatment of mares with hCG for induction of ovulation within 48h before ovulation but not on day 5 of the luteal phase stimulates progestin secretion and may enhance conceptus development via stimulation of endometrial function during early pregnancy.