BACKGROUND Current markers of iron deficiency tend to be less reliable in pregnancy. OBJECTIVE Our aim was to study the usefulness of soluble serum transferrin receptor (sTfR) as a marker for iron deficiency during early and late gestation and to define iron status in 254 pregnant Swedish women. METHODS We performed a cross-sectional and longitudinal evaluation of sTfR in comparison with concentrations of serum ferritin and hemoglobin in blood collected around gestational weeks 11 and 36. RESULTS The specificity of sTfR was 100%. The sensitivity in relation to both anemia and depleted iron stores was approximately 70%, but this figure is less reliable because of few samples. sTfR in early pregnancy was low: 11% of women had a value below the reference interval. sTfR increased significantly from early to late pregnancy even in the group of women with persisting iron stores. In late pregnancy, 14% of women developed tissue iron deficiency and 5% had iron deficiency according to a combination of all 3 markers. CONCLUSIONS sTfR seems to be a specific and sensitive marker of iron deficiency in pregnancy and may have advantages over serum ferritin and hemoglobin. The low sTfR concentration in early gestation seems to be caused by reduced erythropoiesis, whereas the increase from early to late pregnancy reflects increased erythropoiesis, and in case of iron deficiency, also tissue iron deficiency. Further studies are needed to verify whether decreased erythropoiesis reduces the possibility of detecting iron deficiency during early gestation by sTfR.