Objective: To study the correlation between third-trimester serum ferritin concentration and hemoglobin and red cell indices to select the best hematologic characteristic to identify women who need iron therapy.Methods: In a prospective study, blood was drawn from pregnant women with hemoglobin above 10 g/dL, and thalassemia trait excluded at booking, at 28-30 weeks' gestation to study the correlation (Spearman ρ value) between serum ferritin concentration and hemoglobin level, mean corpuscular volume, mean cell hemoglobin, mean cell hemoglobin concentration, and hematocrit. The best relationship was identified using receiver operating characteristic (ROC) curves.Results: Serum ferritin concentration correlated significantly with hemoglobin (ρ = .211), mean corpuscular volume (ρ = .332), mean cell hemoglobin (ρ = .304), and hematocrit (ρ = .199). The area under the ROC curve was largest for hemoglobin.Conclusion: Serum ferritin concentration at the early third trimester correlated best with hemoglobin level. If a hemoglobin level of 11 g/dL or below (25% of all patients) was used as the cutoff, 64% of women who needed iron therapy were identified.