Predicting delivery date by ultrasound and last menstrual period in early gestation

Objective: To compare last menstrual period and ultrasonography in predicti ng delivery date. Methods: We used ultrasound to scan 17,221 nonselected singleton pregnancie s at 8-16 completed weeks. The last menstrual period (LMP) was considered c ertain in 13,541 and uncertain in 3680 cases. The duration of pregnancy fro m the scan to the day of spontaneous delivery was predicted by crown-rump l ength, biparietal diameter (BPD), and femur length (FL) using linear regres sion models, and the results were compared with estimates based on LMP. Results: At all gestational ages, ultrasound was superior to certain LMP in predicting the day of delivery by at least 1.7 days. When deliveries befor e 37 weeks were excluded, crown-rump length measurement of 15-60 mm (corres ponding to 8-12.5 weeks) had the lowest prediction error of 7.3 days. After that time, BPD (at least 21 mm) showed a similar error (7.3 days) and was more precise than crown-rump length. Femur length was slightly less accurat e than crown-rump length or BPD. Regression models using a combination of a ny two or three ultrasonic variables did not improve accuracy of prediction . When ultrasound was used instead of certain LMP, the number of postterm p regnancies decreased from 10.3% to 2.7% (P < .001). Conclusion: Ultrasound was more accurate than LMP in dating, and when it wa s used the number of postterm pregnancies decreased. Crown-rump length of 1 5-60 mm was superior to BPD, but then BPD (at least 21 mm) was more precise . Combining more than one ultrasonic measurements did not improve dating ac curacy. (Obstet Gynecol 2001;97:189-94. (C) 2001 by The American College of Obstetricians and Gynecologists.).