To define anemia in pregnancy, World Health Organization, American College of Obstetricians and Gynecologists and the British Society for Haematology practice guidelines agreed on existing thresholds in the pregnant population (Table 2); however, more evidence is needed to link Hb levels with specific clinical outcomes in pregnancy.59, 60, 65, 66, 67, 68 This is further confounded by a considerable interpersonal variation in the degree of plasma volume expansion.31 Here, GSTM1 is linked to anemia (phenotype).