Effects of hemoglobin levels during pregnancy on adverse maternal and infant outcomes: a systematic review and meta-analysis

Maternal anemia affects approximately 56 million women worldwide and increases the risk of adverse pregnancy outcomes. Our study aimed to summarize the evidence for the association between maternal hemoglobin (Hb) concentrations and maternal or infant outcomes, evaluating it in a continuous manner. In this systematic review and meta-analysis, we conducted an electronic search on PubMed, Embase, CINAHL, and Web of Science from inception to April 19, 2017, and further updated to November 21, 2018, applying subject heading terms related to pregnant women with anemia. We included 117 studies with 4,127,430 pregnancies. Maternal anemia increased the risk of low birth weight (odds ratio (OR), 1.65; 95% confidence interval (CI): 1.45-1.87), preterm birth (PTB) (OR, 2.11; 95% CI: 1.76-2.53), perinatal mortality (PNM) (OR, 3.01; 95% CI: 1.92-4.73), stillbirth (OR, 1.95; 95% CI: 1.15-3.31), and maternal mortality (OR, 3.20; 95% CI: 1.16-8.85). A nonlinear relationship was found between maternal Hb and adverse maternal and infant outcomes. The OR of outcomes such as PTB, small-for-gestational age, PNM, preeclampsia, gestational hypertension, and postpartum hemorrhage was increased by two to three times. Assessing Hb as a continuous variable is important to determine the associated risk of adverse outcomes with decreasing or increasing levels.