[Assessment of gestational age using ultrasound--can the method be improved?]

In Norway, ultrasound measurement of the fetal biparietal diameter is used to determine the date of confinement according to Eik-Nes & Grøttum's method. We aimed to evaluate the precision of this method. 8,029 women with singleton pregnancy and spontaneous vaginal delivery were arranged in groups according to gestational age at the time of the ultrasound examination. The precision of the biparietal diameter measurement for predicting the date of birth was determined for each group. An alternative method by Altman & Chitty was also tested on the population. Eik-Nes & Grøttum's method predicts well the date of confinement if the biparietal diameter is measured at 17-20 weeks. Measurements at an earlier stage predict the date of birth with less confidence, particularly during completed gestational weeks 13-16, when the mean error reaches four days. Altman & Chitty's chart seemed to perform more evenly for the various gestational ages, but was systematically shifted by 3-4 days when used on our population. Eik-Nes & Grøttum's chart for assessing gestational age should preferably not be used before 17 weeks of gestation. Introducing new charts based on a different population is not a good option. New charts based on a Norwegian population are needed.