Systematic misclassification of gestational age by ultrasound biometry: implications for clinical practice and research methodology in the Nordic countries

Historically, pregnancy dating has been based on self-reported information on the first day of the last menstrual period. In the 1970s, ultrasound biometry was introduced as an alternative for pregnancy dating and is now the leading method in Nordic countries. The use of ultrasound led to a reduction of post-term births and fewer inductions, and is considered more precise than last menstrual period-based methods for pregnancy dating. Nevertheless, differences in early growth and specific situations, such as maternal obesity, can render its estimates less precise, leading to gestational age misclassification. Clinical implications of ultrasound dating include effect on timely induction in case of post-term pregnancies, treatment with corticosteroids in cases of anticipated preterm delivery and decision on viability in cases of extreme prematurity. Furthermore, gestational age misclassification may influence the numbers and the magnitude of some adverse perinatal outcomes, closely related to gestational age, which are recorded in the Nordic birth registers.