Progesterone and equine chorionic gonadotropin concentrations around the time of pregnancy loss in mares impregnated by donkeys or stallions

It is concluded that the incidence of pregnancy loss is higher in mares carrying a mule embryo than in mares carrying a horse embryo. However, this is not due to the low progesterone concentrations associated with the premature regression of the endometrial cups that occurs in mares with interspecific pregnancy.