Changes in the hemostaticsystem during pregnancy in heifers and heifer calves were studied, reference intervals for the thrombocytogram and the blood coagulation system during "physiological pregnancy" were established, the dependence of blood parameters on progesterone concentration and pregnancy trimester was assessed. The work was performed on heifer calves(pregnant) and heifers of the Holstein breed, from whom blood was taken for research. It was found that during pregnancy, the number of platelets and platelet crit in the thrombocytogram decreases by 3.36 and 3.62 times (P<0.05), but the number of large platelets increases by 39.87% (P<0.05). The clotting tendency of blood is determined by the variability of parameters associated with the fibrinogenesis process: thrombin time decreases by 19.88% (P<0.05) and the concentration of fibrinogen increases by 38.98% (P<0.05). This is facilitated by a decrease in antithrombin III level by 37.04%. The data obtained during a physiological pregnancy can be used as normative when assessing the condition of animals, which will allow timely to detect various abnormalities in the hemostatic system and to carry out appropriate preventive measures.

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