Determination of gestational age without an accurate last menstrual period is difficult. In the later stages of pregnancy, the correlation between fetal biparietal diameter (BPD), as determined by ultrasound, and gestational age is compromised by normal biologic variation in fetal growth rates. The fetal crown-rump measurement (CRM) by gray-scale sonography has been described but has not been evaluated using real-time ultrasound. Currently available tables for determining gestational age from the fetal CRM were evaluated using a linear array real-time ultrasound system. A normal obstetric population with known menstrual histories, delivery dates, and Dubowitz scores at delivery was used for evaluation. On the basis of this study, Robinson's data listed as “regression analysis values” are suggested for application to real-time ultrasound. Alternatively, the equation generated from data on the above 86 patients could be used and further evaluated. The fetal CRM appears to be a valid and accurate assessment of gestational age.