Fig. 2.

Method for estimating gestational age, all liveborn singleton births, 2000–2020. Note: The distribution in gestational age at birth by method of estimation. Dating from ultrasound and last menstrual period (LMP) have assumed a gestational length of 280 days. Dating by time of LMP overestimates the proportion with longer gestation. The date recorded in clinical records is in recent years based on ultrasound dating, but assumes a gestation of 279 day in some electronic health record systems. Before introduction of electronic health records, or if it not available, there is no underlying information about methods used to estimate gestational age in the clinical records