A new formula for calculating weight in the fetus of < or = 1600 g.
OBJECTIVE
To develop and test a new formula for estimating weight in the fetus of < or = 1600 g.


METHODS
A formula for sonographic estimation of fetal weight was produced retrospectively from 84 singleton fetuses with a birth weight of < or = 1600 g, examined sonographically within 1 week before delivery. Exclusion criteria were multiple pregnancy, intrauterine death and major structural or chromosomal anomalies. The new formula was then compared prospectively in an evaluation group of fetuses (n = 87) with six currently available equations for estimating weight in the preterm fetus.


RESULTS
Stepwise regression analysis with gestational age (in days) and fetal biometric parameters was employed to yield the best-fit formula for predicting fetal weight at birth. The new formula (estimated fetal weight = 5381.193 + 150.324 x head circumference + 2.069 x femur length3 + 0.0232 x abdominal circumference3-6235.478 x log(head circumference)) proved to be superior to established equations. The lowest mean +/- SD absolute error was 66.2 +/- 59 g and the lowest mean absolute percentage error was 7.1 +/- 5.9% SD when studied prospectively in the evaluation group. With the new formula, 48.3% of estimates fell within +/-5% of the actual weight at birth, 73.6% fell within +/-10%, 90.8% fell within +/-15% and 95.4% fell within +/-20%.


CONCLUSIONS
Our new formula is relatively easy to use and needs no adjustment to weight centiles or to fetal lie. It allows reliable weight estimation in the fetus < or = 1600 g.
Department of Obstetrics and Fetal Medicine, Centre for Obstetrics and Gynaecology, Germany. ralf.schild@gyn.imed.uni-erlangen.de
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RL
Schild
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Fell
R
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Fimmers
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Gembruch
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Hansmann
D001835:Body Weight; D005260:Female; D005333:Fetus / Q000033:anatomy & histology*; D006801:Humans; D007231:Infant, Newborn; D019102:Infant, Very Low Birth Weight; D011247:Pregnancy; D012044:Regression Analysis; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal