Incorporation of femur length leads to underestimation of fetal weight in asymmetric preterm growth restriction

Abstract

Objective To review the performance of a variety of biometry formulae for estimated fetal weight (EFW) in the management of severely growth restricted fetuses with abnormal umbilical artery Doppler at a single perinatal institution. Methods Forty-three pregnancies were retrospectively reviewed. Inclusion criteria were: chromosomally/ structurally normal fetus; complete ultrasound biometry at ≤7 days from delivery; EFW <10th centile; absent/reversed end-diastolic flow in the umbilical arteries; and delivery at <32 + 6 weeks. EFW accuracy and precision were compared among nine formulae utilizing combinations of head circumference (HC), biparietal diameter (BPD), abdominal circumference (AC) and femur length (FL) measurements. Results Twenty-six (60.5%) fetuses showed asymmetric growth (HC/AC ratio > 95th centile). Analysis of the systematic and random errors associated with each formula showed that the birth weight of asymmetricallygrown fetuses was most closely approximated by the Hadlock equation that utilized BPD and AC measurements only. The birth weight of symmetrically-grown fetuses was most closely approximated by EFW derived from Hadlock equations that utilized ≥ three biometry measurements, including FL. Incorporation of FL into Hadlock formulae led to significant underestimation of birth weight in the fetuses with asymmetric growth (mean percentage error ± SD: EFWFL-AC, -13.3 ± 9.8%; EFWBPD-FL-AC, -10.8 ± 9.8%; EFWHC-FL-AC, -11.8 ± 9.3%; EFWBPD-HC-FL-AC, -11.7 ±} 9.5%; P < 0.001). The same equations were accurate in fetuses with symmetric growth (EFWFL-AC, 3.1 ± 10.0%; EFWBPD-FL-AC, 1.0 ± 8.9%; EFWHC-FL-AC, 0.3 ±) Copyright © 2010 ISUOG. Published by John Wiley & Sons, Ltd.