ESTIMATING THE DATE OF CONFINEMENT - ULTRASONOGRAPHIC BIOMETRY VERSUSCERTAIN MENSTRUAL DATES

OBJECTIVE: Our purpose was to evaluate the clinical implications of cu rrent pregnancy dating policies in a population where routine ultrason ography is performed in the first half of pregnancy. STUDY DESIGN: A t otal of 34,249 computer files of singleton pregnancies that had both ' 'ceriain'' menstrual dates and ultrasonographic biometry were retrieve d from the East Midlands Obstetric Database. The estimated dates of de livery were calculated by five different methods: menstrual dates alon e, ultrasonography alone, or a combination of both with menstrual date s used if the discrepancy with the ultrasonography dales was within 7, 10, or 14 days, respectively. The accuracy of each method in predicti ng the actual date of delivery was calculated. Differences among metho ds were evaluated with nonparametric tests. RESULTS: Compared with use of certain menstrual dates, ultrasonographic dating led to a 70% redu ction in the number of pregnancies considered postterm. Delivery occur red within +/-7 days of the estimated date of confinement in 49.5% cas es when menstrual dates alone were used and in 55.2% if ultrasonograph y alone was used; for +/-10 days, the corresponding figures were 64.1% and 70.3%. Scan dating alone was significantly better in predicting t he actual date of delivery than any of the dating policies taking mens trual dates alone or in combination with ultrasonography. CONCLUSIONS: Even if menstrual dates are considered ''certain,'' there is no advan tage in taking them into consideration for calculating the expected da te of delivery if a dating ultrasonography result is available. Dating by ultrasonographic biometry in the first half of pregnancy results i n a more accurate prediction of the delivery date than using menstrual data alone or in combination with ultrasonography.