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

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Abstract
An association between moderate anemia and poor perinatal outcomes has been found through epidemiologic studies, although available evidence cannot establish this relation as causal. Anemia may not be a direct cause of poor pregnancy outcomes, except in the case of maternal mortality resulting directly from severe anemia due to hypoxia and heart failure. Preventing or treating anemia, whether moderate or severe, is desirable. Because iron deficiency is a common cause of maternal anemia, iron supplementation is a common practice to reduce the incidence of maternal anemia. Nevertheless, the effectiveness of large-scale supplementation programs needs to be improved operationally and, where multiple micronutrient deficiencies are common, supplementation beyond iron and folate can be considered. High hemoglobin concentrations are often mistaken as adequate iron status; however, high hemoglobin is independent of iron status and is often associated with poor health outcomes. Very high hemoglobin concentrations cause high blood viscosity, which results in both compromised oxygen delivery to tissues and cerebrovascular complications. Epidemiologic studies have also found an association between high maternal hemoglobin concentrations and an increased risk of poor pregnancy outcomes. Evidence does not suggest that this association is causal; 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, not necessarily as a sign of adequate iron nutrition, because iron supplementation does not increase hemoglobin higher than the optimal concentration needed for oxygen delivery.