Several prior systematic reviews have examined associations between maternal anemia and birth outcomes.5, 6, 7, 8, 9, 10, 11, 12 Across the studies reviewed, there is high heterogeneity in the definition of anemia, the timing of exposure, etiology of anemia, and in the definitions of normal or healthy reference ranges for Hb for comparison. Here, GSTM1 is linked to anemia (phenotype).