Accurate gestational dating: revisited.

Using first trimester fetal crown-rump length (CRL) measurements as the criterion for gestational dating, rather than menstrual history or neonatal gestational age evaluation after birth, may be a more accurate way of evaluating the accuracy of second or third trimester ultrasonic measurements for gestational age estimation. This hypothesis was tested using two study groups to evaluate the accuracy of second or third trimester ultrasound measurements for the estimation of gestational age. Three hundred twenty normal obstetric patients who had first trimester CRL measurements were evaluated in the first study. Stepwise multiple regression analysis was used to determine the best combination of multiple ultrasonic parameters for calculation of gestational age, using each patient's CRL as his or her own control. The best formula derived from the first set of patients was used in a second set of 138 high-risk patients to determine the accuracy of multiple ultrasonic parameters obtained during the second or third trimester to date gestation accurately. The analysis confirmed the accuracy of multiple ultrasonic parameters for gestational dating and indicated that the use of two parameters (head circumference and femur length) was as accurate as the traditional four-parameter technique.