Comparison of ultrasonic measurement of biparietal diameter and last menstrual period as a predictor of day of delivery in women with regular 28 day-cycles.

Within a randomised trial, 1713 women with regular 28-day cycles were studied to evaluate whether the day of delivery is predicted best from the last menstrual period (LMP) or ultrasonic measurement of the fetal biparietal diameter (BPD). Among 727 women, with a spontaneous onset of labor, where the estimated day of delivery (EDD) was derived from BPD, 91.8% delivered within +/- 14 days and 61.8% delivered within +/- seven days of EDD. Corresponding figures for 741 women where EDD was derived from LMP were 91.6% and 61.1%, respectively. Comparison of EDDs calculated from BPD and LMP in the same woman showed that BPD postponed EDD more than seven days in 18.0% and advanced EDD more than seven days in 1.8%. When the difference in EDD between the two methods was more than seven days the BPD estimate was a better predictor of day of delivery.