Mares have an atypical hormone profile during pregnancy. Systemic progesterone (P4) levels approach zero by day 220 of gestation. Other reduced pregnanes such as 5α-pregnane-3, 20-dione (5α), 5α-pregnane-3β, 20α-diol (βα), 3β-hydroxy-5α-pregnan-3-one (3β) and 20α-hydroxy-5α-prenan-3-one (20α), increase to near μg/mL levels in the peripheral system of the mare until directly before...