Estimated date of delivery from last menstrual period and ultrasound scan: which is more accurate?

The aim of this study was to determine which is the more accurate predictor of the date of delivery for pregnant women in a community-based population: a calculation based on the last menstrual period or a prediction based on the measurement by ultrasound scanning of well-recognized fetal characteristics. A prospective study was conducted of 225 consecutive women reporting their pregnancy in a semi-rural general practice; 106 women were included in the analysis. The results showed that in over 50% of cases the estimated date of delivery derived from the ultrasound scan was more accurate than that derived from the last menstrual period, whatever the discrepancy between the two predictions. It can be concluded that all professionals involved with antenatal care should ignore the estimated date of delivery derived from the last menstrual period once a satisfactory scan has been obtained.