Iron Supplementation and Maternal Red Cell Responses

The decline in Hb in the first trimester is now seen as a normal physiologic event and is the result of expansion of the plasma volume. Overzealous supplementation to prevent this physiological anemia has been associated with risk of poor fetal outcomes in at least one study [9]. The normal nadir of Hb is between 24 and 32 weeks of gestation, after which the Hb concentration again rises to levels similar to that seen in the first trimester. The extent of this Hb readjustment may be affected by iron reserves as the large expansion of the red cell mass in the second trimester and early third trimester usually depletes all iron reserves, and physiologic anemia may now be replaced with nutritional anemia.

Maternal Hb concentration and infant outcomes have a U-shaped curve, with an increased risk for poor outcomes at each end of the distribution [10]. High Hb likely reflects an improper expansion of the plasma volume, as in preeclampsia, with increased infant mortality and morbidity [11]. The variation in the amount of hemodilution is considerable, and makes the relatively simple Hb measurement quite unreliable with regard to diagnosis of iron deficiency anemia. Current target Hb concentrations in each trimester are based on supplementation trials, which suggest that Hb > 110 g/l in first and third trimesters and 105 g/l in the second trimester represent reasonable clinical expectations of lower normal levels [1].

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