Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates.

OBJECTIVE: Our purpose was to evaluate the clinical implications of current pregnancy dating policies in a population where routine ultrasonography is performed in the first half of pregnancy.

STUDY DESIGN: A total of 34,249 computer files of singleton pregnancies that had both "certain" menstrual dates and ultrasonographic biometry were retrieved from the East Midlands Obstetric Database. The estimated dates of delivery were calculated by five different methods: menstrual dates alone, ultrasonography alone, or a combination of both with menstrual dates used if the discrepancy with the ultrasonography dates was within 7, 10, or 14 days, respectively. The accuracy of each method in predicting the actual date of delivery was calculated. Differences among methods were evaluated with nonparametric tests.

RESULTS: Compared with use of certain menstrual dates, ultrasonographic dating led to a 70% reduction in the number of pregnancies considered postterm. Delivery occurred within +/- 7 days of the estimated date of confinement in 49.5% cases when menstrual dates alone were used and in 55.2% if ultrasonography alone was used; for +/- 10 days, the corresponding figures were 64.1% and 70.3%. Scan dating alone was significantly better in predicting the actual date of delivery than any of the dating policies taking menstrual dates alone or in combination with ultrasonography.

CONCLUSIONS: Even if menstrual dates are considered "certain," there is no advantage in taking them into consideration for calculating the expected date of delivery if a dating ultrasonography result is available. Dating by ultrasonographic biometry in the first half of pregnancy results in a more accurate prediction of the delivery date than using menstrual data alone or in combination with ultrasonography.