Longitudinal assessment of hemostatic adaptations in pregnant mares: Evidence of a physiological hypercoagulable state

Pregnancy triggers intricate hemostatic adjustments that support placental maturation and prepare the maternal organism for delivery. We hypothesized that pregnant mares would progressively shift toward a procoagulant state throughout gestation, reflected in changes to coagulation and fibrinolytic markers. To test this hypothesis, 15 clinically healthy Spanish Purebred pregnant mares were monitored monthly from conception to term, alongside 15 non-pregnant controls of similar age and breed. A comprehensive hemostatic profile—including platelet (PLT) count, prothrombin time (PT), activated partial thromboplastin time (aPTT), antithrombin III (ANT-III) activity, fibrinogen (FIB), and D-dimer (D-D) concentration—was assessed longitudinally. During early gestation, PT (11.5 ± 0.1 s) and aPTT (44.8 ± 0.6 s) remained comparable to controls (11.5 ± 0.1 s; 44.8 ± 0.6 s). From mid-gestation onward, PT and aPTT decreased significantly (P < 0.05), reaching 10.8 ± 0.3 s and 41.2 ± 0.9 s in the final month. ANT-III activity declined from 101 ± 1.2% to 86 ± 2.3% (P < 0.05), while controls maintained 99.6 ± 2.1%. FIB increased from 202 ± 18 mg/dL to 291 ± 53 mg/dL (P < 0.05), and D-D rose from 225 ± 12 ng/mL at month 6–495 ± 26 ng/mL at term (P < 0.05). These findings confirm a gestation-dependent shift toward a physiological hypercoagulable state, marked by enhanced coagulation and fibrin turnover without clinical signs of thrombosis. This adaptation appears non-pathological and emphasizes the need for gestation-specific reference intervals.