Corpus Luteum Vascularization and Progesterone Production in Autumn and Winter Cycles of the Mare: Relationship Between Ultrasonographic Characteristics of Corpora Lutea and Plasma Progesterone Concentration in the Last Cycles Before Anestrus

In 20 estrus cycles of 15 mares, Color Doppler ultrasound of corpora lutea and plasma progesterone concentration (P4) was analyzed on days 6, 10, 14, 16, and 18 after ovulation. Progesterone concentration was positively correlated with corpora lutea cross-sectional area (CSA), vascularized area (VA), and index of vascularization (IV = VA/CSA) (P < .0001). Cross-sectional area, VA, and IV in corpora lutea of mares with P4 < 1 ng/mL were significantly lower than in corpora lutea of mares with P4 > 1 ng/mL. Mares with CSA < 3,473 pixels, VA < 25.5 pixels, and IV < 7.6% were prone to express P4 < 1 ng/mL 25.4, 7.9, and 7.6 times more than mares with higher values, respectively. Corpus luteums analyzed parameters differed significantly between last cycles (LCs) of the breeding season and previous cycles until day 14 after ovulation (P < .05). No significant differences were found in P4 between LCs and previous ones.