Hemoglobin Concentration and Pregnancy Outcomes: A Systematic Review and Meta-Analysis

OBJECTIVE To conduct a systematic review and meta-analysis of hemoglobin effect on the pregnancy outcomes. METHODS We searched MEDLINE and SCOPUS from January 1, 1990 to April 10, 2011. Observational studies addressing association between hemoglobin and adverse pregnancy outcomes were selected. Two reviewers independently extracted data. A mixed logistic regression was applied to assess the effects of hemoglobin on preterm birth, low birth weight, and small for gestational age. RESULTS Seventeen studies were included in poolings. Hemoglobin below 11 g/dL was, respectively, 1.10 (95% CI: 1.02-1.19), 1.17 (95% CI: 1.03-1.32), and 1.14 (95% CI: 1.05-1.24) times higher risk of preterm birth, low birth weight, and small for gestational age than normal hemoglobin in the first trimester. In the third trimester, hemoglobin below 11 g/dL was 1.30 (95% CI: 1.08-1.58) times higher risk of low birth weight. Hemoglobin above 14 g/dL in third trimester decreased the risk of preterm term with ORs of 0.50 (95% CI: 0.26-0.97), but it might be affected by publication bias. CONCLUSIONS Our review suggests that hemoglobin below 11 g/dl increases the risk of preterm birth, low birth weight, and small gestational age in the first trimester and the risk of low birth weight in the third trimester.