Low plasma progestin concentration during the early postovulatory phase impairs equine conceptus development in the late preimplantation phase

In horses, low postovulatory progestin concentrations delay downregulation of endometrial progesterone receptors, impairing histotroph composition. The aim of this study was to evaluate conceptus development until placentation in mares under experimentally reduced progestin concentrations in the early postovulatory phase. Eleven mares were inseminated until ovulation and received the prostaglandin F2α agonist cloprostenol (125 µg) once daily for 4 days after ovulation (treatment) or were left untreated (control). Conceptus growth and development was analysed daily from Day 10 after ovulation by ultrasound. On Day 34 conceptuses were recovered and underwent microcomputed tomography. Statistical comparison was performed by non-parametric Wilcoxon test or Chi2. First visualisation of embryonic vesicle, embryo proper and heartbeat did not differ. Conceptus fixation in the uterus occurred earlier in controls than in conceptuses recovered from treatment pregnancies (P < 0.05). Embryos from treatment pregnancies were smaller (P < 0.05) and lighter (P < 0.05) than controls. In the lung, the mean number of bronchi was lower in embryos from treatment pregnancies than from control pregnancies (P < 0.05). Persistence of communication between the cardiac ventricles was observed more often in embryos from treatment than control pregnancies (P < 0.05). In conclusion, subphysiological progestin concentrations in the early postovulatory period impaired conceptus development before placentation in horses.