Review of the reproductive endocrinology of the pregnant and parturient mare

Analytical advancements, especially methods using gas or liquid chromatography tandem mass spectrometry, have allowed more specific and reliable measurement of multiple steroid hormones in the plasma of mares throughout gestation and the periparturient period. Data such as these will form the central focus of this review. The comprehensive analyses possible with liquid chromatography tandem mass spectrometry illuminate the key physiological and developmental transitions that make equine gestation unique. Weeks 6 to 20 encompass endometrial cup formation and equine chorionic gonadotropic secretion that stimulates primary corpora lutea and induces formation of secondary luteal structures. The period is defined by increased progesterone, 17OH-progesterone, and androstenedione secretion, providing substrate feeding the rise in estrone sulfate that can be used as an aid in the diagnosis of pregnancy. The 5α-reduced metabolite of progesterone, dihydroprogesterone (DHP), parallels progesterone secretion during this period at less than half the concentration. After week 12, progesterone declines, but DHP concentrations continue to increase, exceeding progesterone by week 16, thereby defining the luteo-placental shift in pregnane synthesis from ovarian to primarily placental thereafter. The growth of fetal gonads begins around week 14 and is defined by increasing dehydroepiandrosterone, among other androgens, which fuels placental estrogen secretion, functioning as a true fetoplacental unit. Metabolites of DHP (including allopregnanolone) dominate in late gestation, some exceeding DHP by 10-fold near term. However, all major pregnanes decrease from 3 days before foaling, when fetal cortisol is reportedly rising. Though unique, equine pregnancy and parturition share many features in common with those seen in human pregnancy and birth.