The present study will determine if oral administration of 60 mg of iron as ferrous sulfate during the second trimester of pregnancy is followed by the appearance of non-transferrin-bound iron two hours later. The investigators will examine the influence of a variety of factors, including iron status and previous use of iron supplements. Detection of non-transferrin-bound iron after administration of an iron supplement to pregnant women would be an important new observation that could lead to re-evaluation of current methods of iron supplementation.