he Biostatistics Center served as coordinating center for the trial of Routine Antenatal Diagnostic Imaging with Ultrasound (RADIUS); an office-based, multi-center clinical trial of ultrasound use in low risk pregnancies. RADIUS was a randomized clinical trial designed to evaluate the efficacy of routine prenatal ultrasound use on perinatal outcome in obstetrical participants at low risk for perinatal problems. Over a 3.5 year period in 109 primary care sites in 5 geographic regions of the US, RADIUS randomized a total of 15,530 low-risk obstetrical patients. Participants were randomized to one of two groups: selective use of ultrasound on the basis of clinical judgment or two routine antenatal ultrasounds which were scheduled at 16 to 22 and 31 to 35 week gestation. Pregnancy outcome was abstracted from antenatal medical records and inpatient hospital records. RADIUS concluded that screening ultrasound did not improve perinatal outcome as compared with the selective use of ultrasonography on the basis of clinical judgment (The New England Journal of Medicine 1993;329:821-827). Funded by NICHD (U01-HD-19897), 1987-1993.