High and low hemoglobin levels during pregnancy: Differential risks for preterm birth and small for gestational age

Objective: To examine the association of maternal hemoglobin during pregnan cy with preterm birth and small for gestational age (SGA). Methods: We performed a retrospective cohort analysis of hemoglobin and bir th outcome among 173,031 pregnant women who attended publicly funded health programs in ten states and delivered a liveborn infant at 26-42 weeks' ges tation. We defined preterm as less than 37 weeks' gestation and SGA as less than the tenth percentile of a US fetal growth reference. Results: Risk of preterm birth was increased in women with low hemoglobin l evel in the first and second trimester. The odds ratio (OR) for preterm bir th with moderate-to-severe anemia during the first trimester (more than thr ee standard deviations [SD] below reference median hemoglobin, equivalent t o less than 95 g/L at 12 weeks' gestation) was 1.68 (95% confidence interva l [CI] 1.29, 2.21). Anemia was not associated with SGA. High hemoglobin lev el during the first and second trimester was associated with SGA but not pr eterm birth. The ORs for SGA in women with very high hemoglobin level durin g the first and second trimester (more than three SDs above reference media n hemoglobin, equivalent to greater than 149 g/L at 12 weeks' gestation and greater than 144 g/L at 18 weeks') were 1.27 (95% CI 1.02, 1.58) and 1.79 (95% CI 1.49, 2.15), respectively. Conclusion: These data highlight the importance of considering anemia and h igh hemoglobin level as indicators for adverse pregnancy outcome. An elevat ed hemoglobin level (greater than 144 g/L) is an indicator for possible pre gnancy complications associated with poor plasma volume expansion, and shou ld not be mistaken for good iron status. (Obstet Gynecol 2000;96:741-8. (C) 2000 by The American College of Obstetricians and Gynecologists.).