This prospective study examines the reliability of ultrasonically measured head/abdominal (H/A) ratios in detecting uteroplacental insufficiency (UPI)-related intrauterine growth retardation (IUGR) in 47 patients. Normal H/A ratios were found in 37 (79%) fetuses. None of these infants was classified as small for gestational age (SGA) on newborn pediatric evaluation. Ten fetuses (21%) had H/A ratios greater than +2 SD for gestational age. On this basis, UPI-related IUGR was correctly predicted in all 10 infants. It is concluded that the H/A ratio is a sensitive indicator of UPI-related IUGR. Moreover, this technique is particularly valuable in patients referred for initial scanning after 30 weeks' gestation when it is difficult to distinguish accurately between true IUGR and an "error in dates" on the basis of biparietal diameter (BPD) measurements alone.