A comparison of the ability of a sonographically measured biparietal diameter and the last menstrual period to predict the spontaneous onset of labor.

To study whether the estimated date of confinement (EDC) for women with optimal menstrual histories is predicted best from the last menstrual period (LMP) or biparietal diameter (BPD), 2320 women had the EDC estimated by both LMP and BPD measured sonographically in the second trimester. Those who delivered after the spontaneous onset of labor are reported here. Compared with the EDC calculated from the LMP, the EDC calculated from the BPD was postponed more than 1 week for 406 women (17.5%) and advanced more than 1 week for 56 (2.4%). For these women, the BPD estimate of EDC turned out to be closer to the day of delivery in 264 cases, whereas the LMP estimate was closer in 125 (P less than .001). For 73 women, the estimates were equally good. For 80.1% of the women, the two estimates of EDC differed by 1 week or less; in these cases, the estimates were equally good in predicting the day of delivery. We conclude that the BPD is a better predictor of the spontaneous onset of labor than is the LMP.