Predicting date of birth and examining the best time to date a pregnancy

ObjectiveTo compare the estimated date of birth (eDOB) from the last menstrual period (LMP) and ultrasound scans at varying gestations (< 70, 70–106, 110–140, 141–196, and 200–276 weeks) with the actual date of birth (aDOB).MethodsIn a retrospective study, data were analyzed from 18 708 women with spontaneous labor who delivered a single neonate without major anomalies in a local health district in Australia between 2007 and 2011. Data were sourced from a computerized population birth database. The study outcomes were duration of pregnancy expressed as total days, and the difference between aDOB and eDOB by dating method.ResultsOnly 5% of births occurred on the eDOB, regardless of the dating method or timing of the dating. Approximately 66% of births occurred within 7 days of the eDOB, and there was little difference among the ultrasound examinations performed at varying gestational weeks. The ultrasound scans at 110–140 weeks of gestation performed as well as ultrasound scans conducted at other gestational ages.ConclusionOn a population basis, there were no meaningful differences in the prediction of date of birth by ultrasound scan date. An early dating scan (≤ 10 weeks) is unnecessary if LMP is reliable.