Ultrasound dating at 12-14 or 15-20 weeks of gestation? A prospective cross-validation of established dating formulae in a population of in-vitro fertilized pregnancies randomized to early or late dating scan.

OBJECTIVES: To determine the accuracy of established ultrasound dating formulae when used at 12-14 and 15-20 gestational weeks.

METHODS: A total of 363 singleton pregnancies conceived after in-vitro fertilization (IVF) and randomized to a dating scan at 12-14 or 15-20 gestational weeks were studied. 'True' gestational age was calculated on the basis of the day of oocyte retrieval and was compared to gestational age calculated on the basis of seven dating formulae including the fetal biparietal diameter (BPD) and three dating formulae including BPD and fetal femur length (FL). The mean of the differences between estimated and true gestational age (systematic measurement error) and their SD (random measurement error) were calculated for each formula.

RESULTS: Three formulae showed systematic errors of less than -0.7 days at both early and late scanning. Two formulae overestimated gestational age at both early and late scanning by 5.7 and 3.1 vs. 2.3 and 2.8 days, respectively, while five formulae manifested very different systematic errors at early and late scanning. The formulae used for clinical management underestimated gestational age by a mean of 3 days when dating was performed at 12-14 weeks, and by a mean of 0.8 days when dating was done at 15-20 weeks. The random error was on average 1 day less when the scan was carried out early (2 vs. 3 days; P < 0.0005). Mean true gestational age at delivery in IVF pregnancies with spontaneous start of labor was 279 days (SD 12.9); excluding preterm deliveries it was 281 days (SD 8.1).

CONCLUSIONS: Ultrasound dating formulae originally intended for use in the middle of the second trimester do not necessarily perform well when used for dating earlier in gestation. The systematic and random error of any dating formula must be assessed for the gestational age interval in which the formula is intended to be used.