Because of recent challenges in the literature regarding the validity of the older crown-rump length (CRL) data developed with conventional static-image ultrasound scanners, the authors evaluated the relationship between CRL and menstrual age of fetuses in a population of 416 patients with good menstrual dates. By using a variety of commercially available transabdominal and transvaginal real-time ultrasound probes, the authors demonstrated that measurements can be made successfully for CRLs varying in size from 2 mm to 12 cm. Regression analysis of the data resulted in development of a new table for predicting menstrual age of fetuses on the basis of CRL measurements obtained between 5 and 18 weeks gestation. Although the magnitude of the raw residuals increased over time, the variability in predicting menstrual age was demonstrated to be relatively constant at +/- 8% (2 standard deviations) when expressed as a percentage of the predicted value. The accuracy in predicting menstrual age from CRL after 14 weeks was equivalent to but not better than conventional measurements such as biparietal diameter and femur length.