The ultrasonic measurement of fetal crown-rump length as a method of assessing gestational age. Br Obstet Gyneaecol 1976;83:417 Accuracy of ultrasound in predicting gestational.age is +/- 7 days for 12-18 weeks: Reece EA, Gebrielli S, Degennaro N, et al. Dating through pregnancy: a measure of growing up. Obstet Gynecol Surv. 1989;44:544.