Abstract
Background. In Norway, routine ultrasound examination with assessment of gestational age was included in the standard antenatal care program in 1985. Until presently, a dating chart introduced in 1983 has been used throughout the country. New national normal values are now developed. The aim of the study was to compare the performance of the two sets of normal values; both are based on measurement of fetal biparietal diameter in the second trimester. Material and method. We analyzed a cohort of 11,238 singleton deliveries with spontaneous onset of labor. All had reliable last menstrual period and all had a routine ultrasound examination in the second trimester with measurement of fetal biparietal diameter. We calculated the expected day of delivery both according to the old (method A) and the new (method B) reference values. Results. The prediction error (the difference between observed and expected day of delivery) for the last menstrual period method was −0.1 days, for method A −3.5 days, and for method B −0.7 days. The prediction error for method A increased up to 7 days when ultrasound measurements were obtained in gestational week 14–15. Conclusion. The old ultrasound method leads to a systematic underestimation of fetal age, and the prediction error increases notably when the ultrasound measurements are performed in the early second trimester. Gestational age is more precisely assessed with the new reference values.