Comparison of Errors of 35 Weight Estimation Formulae in a Standard Collective

Abstract

Issue: The estimation of foetal weight is an integral part of prenatal care and obstetric routine. In spite of its known susceptibility to errors in cases of underweight or overweight babies, important obstetric decisions depend on it. In the present contribution we have examined the accuracy and error distribution of 35 weight estimation formulae within the normal weight range of 2500–4000 g. The aim of the study was to identify the weight estimation formulae with the best possible correspondence to the requirements of clinical routine.

Materials and Methods: 35 clinically established weight estimation formulae were analysed in 3416 foetuses with weights between 2500 and 4000 g. For this we determined and compared the mean percentage error (MPE), the mean absolute percentage error (MAPE), and the proportions of estimates within the error ranges of 5, 10, 20 and 30 %. In addition, separate regression lines were calculated for the relationship between estimated and actual birth weights for the weight range 2500–4000 g. The formulae were thus examined for possible inhomogeneities.

Results: The lowest MPE were achieved with the Hadlock III and V formulae (0.8 %, STW 9.2 % or, respectively, −0.8 %, STW 10.0 %). The lowest absolute error (6.6 %) as well as the most favourable frequency distribution in cases below 5 % and 10 % error (43.9 and 77.5) were seen for the Halaska formula. In graphic representations of the regression lines, 16 formulae revealed a weight overestimation in the lower weight range and an underestimation in the upper range. 14 formulae gave underestimations and merely 5 gave overestimations over the entire tested weight range.

Conclusion: The majority of the tested formulae gave underestimations of the actual birth weight over the entire weight range or at least in the upper weight range. This result supports the current strategy of a two-stage weight estimation in which a formula is first chosen after a pre-estimation of the weight range.

Zusammenfassung

Fragestellung: Die fetale Gewichtsschätzung ist integraler Bestandteil der Schwangerenvorsorge und des geburtshilflichen Alltags. Trotz ihrer bei unter- und übergewichtigen Kindern bekannten Fehleranfälligkeit, hängen wichtige geburtshilfliche Entscheidungen von ihr ab. In der vorliegenden Arbeit wird die Genauigkeit und die Fehlerverteilung von 35 Gewichtsformeln innerhalb des normalen Gewichtsbereichs von 2500–4000 g untersucht. Ziel der Untersuchung war es, Gewichtsformeln zu finden, die den Anforderungen des klinischen Alltags bestmöglich entsprechen.

Material und Methodik: 35 klinisch etablierte Gewichtsschätzformeln wurden an 3416 Feten mit einem Gewicht zwischen 2500 und 4000 g analysiert. Hierbei wurden der mittlere prozentuale Fehler (MPF), der mittlere absolute prozentuale Fehler (MAPF), der Anteil der Schätzungen innerhalb eines Fehlerbereichs von 5, 10, 20 und 30 % ermittelt und verglichen. Darüber hinaus wurden für den Zusammenhang von Schätz- zu tatsächlichem Geburtsgewicht getrennte Regressionsgeraden für den Gewichtsbereich 2500–4000 g berechnet. Die Formeln wurden somit auf eine mögliche Inhomogenität überprüft.

Ergebnisse: Der kleinste MPF wurde mittels den Hadlock-III- und -V-Formeln erzielt (0,8 %, STW 9,2 % bzw. −0,8 %, STW 10,0 %). Den geringsten absoluten Fehler (6,6 %) sowie die günstigste Häufigkeitsverteilung bei Fällen unter 5 % und 10 % Fehler (43,9 und 77,5) wies die Halaska-Formel auf. In der grafischen Darstellung der Regressionsgeraden zeigen 16 Formeln eine Gewichtsüberschätzung im unteren Gewichtsbereich und eine -unterschätzung im oberen Bereich. 14 Formeln unterschätzen und lediglich 5 Formeln überschätzen über den getesteten Gewichtsbereich.

Schlussfolgerung: Die Mehrheit der Formeln unterschätzt im vollständigen oder zumindest oberen Gewichtsbereich das tatsächliche Geburtsgewicht. Die Ergebnisse unterstützen aktuelle Ansätze eines 2-stufigen Vorgehens der Gewichtsschätzung, bei der die Formel erst nach Voreinschätzung des Gewichtsbereichs gewählt wird.

Key words

biometry - foetal weight estimation - regression formula

Schlüsselwörter

Biometrie - fetale Gewichtsschätzung - Regressionsformel

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