First trimester crown-rump length (CRL) followed by a second-trimester biparietal diameter (BPD) were sonographically measured in 27 patients. Each value was independently used to prospectively predict an estimated date of confinement. The actual delivery date was compared with the estimated dates of confinement predicted by the CRL and the BPD. The mean error in predicting the actual date of delivery by CRL was 7.73 days as opposed to 7.65 days with the BPD. This difference is not statistically significant (p greater than 0.9). In both methods there was a greater tendency to overestimate the actual date of delivery. Contrary to widespread belief, the present study shows that a second-trimester BPD is as accurate as a first-trimester CRL in establishing an estimated date of confinement. Additional benefits of second-trimester scanning include the ability to determine placental localization, evaluate amniotic fluid volume, and exclude many congenital abnormalities.