Abstract: Assays of serum ferritin together with hemoglobin, serum iron and total iron binding capacity （TIBC） were used to assess iron stores in 94 cases of healthy pregnant women. 99 non-pregnant women and 60 female patients with simple iron-deficiency anemica served as the controls. The value of serum ferritin indicating depletion stores in iron-defieiency anemca was ≤14ng/ml, which was decreased to 10.4ng/ml in late pregnancy, because of the hemodilution caused by 35%increase of plasma volume.The mean serum ferritin value was 48.9ng/ml in 12 women with gestation of ≤24 weeks, slightly higher than the normal contral; but it desceanded to 10.9ng/ml in 82 cases with gestation≥28 weeks, among which 61% was ≤10.4ng/ml. If both serum ferritin ≤10.4ng/ml and HB ≤11.0 g/dl were used as the criteria for iron-deficiency anemia during pregnancy, the incidence was 36.1% in this series. It suggests that without iron supplements there will be 61% pregnent women losing their iron stores and 36% having iron-deficiency anemia duing late pregnancy. So supplemantary iron is necessary. Since serum iron concentration and TIBC are always increased during pregnancy it can’t reflect the iron balance sensitively. It is evident that serum ferritin determination is at present the best single measure-ment to detect iron deficiency during pregnancy