P17.09: Differences in ultrasonically estimated gestational age of extremely preterm infants when using various dating formulas

Abstract

BACKGROUND: Gestational age (GA) is one of the most important obstetric factors and prediction of date of delivery is usually based on ultrasonographic measurements of the fetus. The estimation of GA might be influenced by using various dating formulas. OBJECTIVE: To compare three dating formulas applied to a cohort of extremely preterm infants. METHODS: The study comprised 513 infants born before 27 gestational weeks included in the Swedish national population study (EXPRESS) with available information on mid-trimester ultrasonographically measured biparietal diameter and femur length. Three dating formulas were applied and the GA estimates compared to the clinically reported GA. RESULTS: The mean reported GA was 173.2 days corresponding well to the Persson & Weldner dating formula (173.3) commonly used in Sweden. The mean GA according to the last menstrual period (LMP), Hadlock et al. formula and Mul et al. formula were 176.8, 175.3, and 175.6 days, respectively. The Hadlock et al. and Mul et al. GA estimates differed significantly from that based on the Person & Weldner formula (both p-values <10(-6) ). Among 68 pregnancies with reported duration of 22 weeks, 33 (49%) and 22 (32%), respectively, had duration of 23 weeks or more when GA was calculated according to LMP and according to the Hadlock et al. formula. CONCLUSION: The estimated gestational age among infants before 27 gestational weeks varied significantly depending on the dating formula used for the calculation of the estimated date of delivery, which might influence the clinical management of extremely preterm fetuses and infants. Copyright © 2011 ISUOG. Published by John Wiley & Sons, Ltd.