Modern data on the role of vitamin B12, and its biologically active form holotranscobalamin in particular, in the development of anemia of pregnancy are presented. Information on the vitamin B12 metabolism and its coupling with the folic acid and homocysteine metabolism is supplied. The new automated immunochemiluminescent method for the active vitamin B12 determination is described. Reference intervals for the holotranscobalamin and related indices that can be used for the differential diagnosis of anemia of pregnancy are specified.