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]) were analyzed on days 6, 10, 14, 16 and 18 after ovulation. [P4] was positively correlated with corpora lutea cross sectional area (CSA), vascularized area (VA) and index of vascularization (IV=VA/CSA) (P<0.0001). CSA, 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 <3473 pixels, VA<25.5 pixels and a 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. CLs analyzed parameters differed significantly between last cycles of the breeding season and previous cycles until day 14 after ovulation (P<0.05). No significant differences were found in [P4] between last cycles and previous ones.