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 < or = 7 days from delivery; EFW < 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 > or = 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: EFW(FL-AC), -13.3 +/- 9.8%; EFW(BPD-FL-AC), -10.8 +/- 9.8%; EFW(HC-FL-AC), -11.8 +/- 9.3%; EFW(BPD-HC-FL-AC), -11.7 +/- 9.5%; P < 0.001). The same equations were accurate in fetuses with symmetric growth (EFW(FL-AC), 3.1 +/- 10.0%; EFW(BPD-FL-AC), 1.0 +/- 8.9%; EFW(HC-FL-AC), 0.3 +/- 8.7%; EFW(BPD-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.
Proctor
L K
LK
Maternal-Fetal Medicine Division, Department of Obstetrics & Gynecology, Mount Sinai Hospital, University of Toronto, Ontario, Canada.
Rushworth
V
V
Shah
P S
PS
Keunen
J
J
Windrim
R
R
Ryan
G
G
Kingdom
J
J
D001699:Biometry; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging* / Q000196:embryology / Q000502:physiology; D005317:Fetal Growth Retardation / Q000000981:diagnostic imaging*; D020567:Fetal Weight; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D007234:Infant, Premature; D011247:Pregnancy; D011256:Pregnancy Outcome; D047928:Premature Birth; D012189:Retrospective Studies; D016216:Ultrasonography, Prenatal; D014469:Umbilical Arteries / Q000002:abnormalities / Q000000981:diagnostic imaging* / Q000196:embryology