Significance of an abnormally low or high hemoglobin concentration during pregnancy: special consideration of iron nutrition

An association between moderate anemia and poor perinatal outcomes has been found through epidemiologic studies, although available evidence cannot es tablish this relation as causal. Anemia may not be a direct cause of poor p regnancy outcomes, except in the case of maternal mortality resulting direc tly from severe anemia due to hypoxia and heart failure. Preventing or trea ting anemia, whether moderate or severe, is desirable. Because iron deficie ncy is a common cause of maternal anemia, iron supplementation is a common practice to reduce the incidence of maternal anemia. Nevertheless, the effe ctiveness of large-scale supplementation programs needs to be improved oper ationally and, where multiple micronutrient deficiencies are common, supple mentation beyond iron and folate can be considered. High hemoglobin concent rations are often mistaken as adequate iron status; however, high hemoglobi n is independent of iron status and is often associated with poor health ou tcomes. Very high hemoglobin concentrations cause high blood viscosity, whi ch results in both compromised oxygen delivery to tissues and cerebrovascul ar complications. Epidemiologic studies have also found an association betw een high maternal hemoglobin concentrations and an increased risk of poor p regnancy outcomes. Evidence does not suggest that this association is causa l; it could be better attributed to hypertensive disorders of pregnancy and to preeclampsia. The pathophysiologic mechanism of these conditions during pregnancy can produce higher hemoglobin concentrations because of reduced normal plasma expansion and cause fetal stress because of reduced placental -fetal perfusion. Accordingly, higher than normal hemoglobin concentrations should be regarded as an indicator of possible pregnancy complications, no t necessarily as a sign of adequate iron nutrition, because iron supplement ation does not increase hemoglobin higher than the optimal concentration ne eded for oxygen delivery.