Variation in the use of diagnostic procedures may be due to characteristics of patients, physicians, or their environment. Testing rates for 24-hour urinary estriol levels (EST), diagnostic ultrasound, and antepartum fetal heart rate testing (AFHRT) were examined in 8,527 deliveries from 1975 through 1978. Over the period, utilization of EST remained constant at about 7% of deliveries, while ultrasound increased from 20% to 35% and AFHRT, from 7% to 12%. This increase persisted after stratifying patients on a multivariate confounder score using 45 items of clinical information. Those receiving antenatal care in a hospital-based group practice or a resident-staffed community clinic were more likely to be tested than patients seen in private offices or a prepaid group practice.