Equine Pregnancy and Clinical Applied Physiology

An understanding of the endocrinology of equine pregnancy is helpful when considering administration of supplemental hormones to pregnant mares. We will begin with a basic review and continue on to applying this information in practical therapeutic situations. Progesterone begins to rise after ovulation in diestrus, irrespective of pregnancy status, with the development of the corpus luteum (CL). This initial rise of progesterone is approximately linear over the first several days; by day 5 after ovulation, serum progesterone is up to approximately 4 ng/mL. If the mare is pregnant, progesterone produced by this CL (or corpora lutea if the mare has doubleovulated) maintains the pregnancy. This CL is called the primary CL of pregnancy. Early progesterone secretion is essentially the same, irrespective of whether the mare is pregnant or is in diestrus. If pregnant, luteolysis does not occur as it would in the nonpregnant mare, and this primary CL is maintained and continues to secrete progesterone. Ginther has termed these two phases of progesterone secretion by the primary CL as “output D” and “output 1,” for diestrous production and first luteal response of pregnancy. Embryo migration between days 11 to 15 is necessary for maternal recognition of pregnancy, which keeps the early pregnant mare from returning to estrus. The signal for maternal recognition of pregnancy is poorly understood in the mare, but embryo migration within the uterus is clearly required. At day 16 or 17 after ovulation, the mobile embryo stops migrating within the uterus, and fixation, normally at the base of one of the uterine horns, occurs. Fixation is thought to be caused by a combination of increasing embryo size (diameter) as well as increased uterine tone, possibly caused by estrogen secreted by the embryo. Maternal recognition of pregnancy by the equine uterus prevents prostaglandin release by the endometrium, thus allowing the continued function of the primary CL. Interestingly, by approximately day 30, progesterone production from the primary CL decreases, resulting in somewhat lower circulating serum progesterone