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

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; EEW <10(th) 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 >95(th) centile). Analysis of the systematic and random errors associated with each formula showed that the birth weight of asymmetrically-grown 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 (EFWEL-AC, 3.1 +/- 10.0%; EFWBPD-FL-AC, 1.0 +/- 8.9%; EBWHC-FL-AC, 0.3 +/- 8.7%; EFWBPD-HC-FL-AC, 0.4 +/- 15.5%). Use of the best performing equation (Hadlock 3), which does not include FL, to estimate weight in asymmetrically-grown fetuses over 28 weeks' gestation, would have reduced the proportion of those with an underestimation of fetal weight of > 100 g from nine (50.0%) to three (16.7%). Conclusions Biometry methods that exclude FL should be considered in asymmetric intrauterine growth restriction associated with abnormal umbilical artery Doppler waveforms. Copyright (C) 2010 ISUOG. Published by John Wiley & Sons, Ltd.