This systematic review and meta-analysis examines the associations of maternal hemoglobin concentrations with birth outcomes (low birth weight, preterm birth, small-for-gestational-age, stillbirth, and perinatal and neonatal mortality) and adverse maternal outcomes (postpartum hemorrhage, preeclampsia, and blood transfusion). It accounts for the timing of measurement (preconception and first, second, and third trimesters); etiology of anemia; and cutoff category.