Measurements of circulating transferrin receptor provide a sensitive quantitative index of tissue iron deficiency in otherwise healthy subjects. This investigation was undertaken to examine the diagnostic utility of this new iron index in pregnancy. A battery of iron-related measurements, including serum transferrin receptor concentrations, was performed on 176 women in third-trimester pregnancy who were attending a university prenatal clinic. The mean receptor concentration of 5.96 +/- 2.37 mg/L (+/- 1 SD) did not differ significantly from concentrations in nonpregnant individuals and the frequency distributions were likewise comparable. Elevations in serum receptor greater than 8.5 mg/L occurred only in women with depleted iron stores defined by serum ferritin concentrations. Abnormal concentrations were found in 11 of 13 women with overt iron-deficiency anemia. Our findings indicate that serum receptor concentrations are not influenced by pregnancy per se and are a sensitive index of iron deficiency. By combining serum receptor and serum ferritin measurements, the entire spectrum of iron status in pregnancy can be assessed.