Sonographic Weight Estimation in Small-for-Gestational-Age Fetuses

Abstract

Purpose: To determine the accuracy of sonographic weight estimation (WE) for small-for-gestational-age (SGA) fetuses, and to further differentiate the evaluation between symmetric and asymmetric SGA fetuses.

Materials and Methods: The accuracy of WE in SGA fetuses (n = 898) was evaluated using 14 sonographic models and was further differentiated between symmetric (n = 750) and asymmetric (n = 148) SGA fetuses. SGA fetuses were considered to be asymmetric with a head circumference to abdominal circumference ratio above the 95th percentile. The accuracy of the different formulas was compared using means of percentage errors (MPE), medians of absolute percentage errors (MAPE), and proportions of estimates within 10 % of actual birth weight.

Results: Results for the subgroup of asymmetric SGA fetuses differed significantly from the subgroup of symmetric SGA fetuses. MPE values were closer to zero with most of the formulas in the asymmetric SGA group. Apart from the Siemer, Shepard, Merz and Warsof equations, all formulas showed an underestimation of fetal weight in asymmetric SGA fetuses. In contrast, in the symmetric SGA group, all of the formulas commonly used for fetuses in a normal weight range showed a systematic overestimation of fetal weight. Overall the best accuracy was achieved by using the Sabbagha equation (MPE 1.7 %; SD 9.0 %; MAPE: 6.0).

Conclusion: An accurate WE in SGA fetuses is feasible using the Sabbagha formula. However, one has to be aware of the significant differences in WE between symmetric and asymmetric SGA fetuses.

Zusammenfassung

Ziel: Ziel dieser Studie war die Evaluation der Genauigkeit der sonografischen Gewichtsschätzung bei Small for Gestational Age (SGA) Feten, sowie deren Differenzierung zwischen symmetrisch und asymmetrisch kleinen Feten.

Material und Methoden: Die Genauigkeit der sonografischen Gewichtsschätzung wurde mittels 14 Gewichtsschätzungsformeln in einer Gruppe von 898 SGA-Feten ermittelt. Dabei wurde zwischen Feten mit symmetrischer (n = 750) und asymmetrischer (n = 148) SGA-Entwicklung unterschieden. Feten mit einer Kopf zu Abdomen Ratio über der 95. Perzentile wurden als asymmetrisch klassifiziert. Um die Genauigkeit der verschiedenen Formeln zu testen und zu vergleichen, wurden folgende Parameter berechnet: mittlerer prozentualer Fehler (MPE), Median des absoluten Fehlers (MAPE) und Prozentsatz an Gewichtsschätzungen innerhalb von 10 % des tatsächlichen Geburtsgewichtes.

Ergebnisse: Die Ergebnisse zwischen den Gruppen der symmetrischen und asymmetrischen SGA-Feten unterschieden sich signifikant. Die MPE Werte der asymmetrischen SGA Gruppe lagen bei fast allen Formeln näher bei null. Mit Ausnahme der Siemer-, Shepard-, Merz- und Warsof-Modelle, zeigten alle anderen Formeln bei asymmetrisch kleinen Feten eine Unterschätzung des Geburtsgewichtes. In der Gruppe der symmetrisch kleinen Kinder zeigten dagegen alle Formeln, die üblicherweise zur Gewichtsschätzung bei normalgewichtigen Feten verwendet werden, eine systematische Überschätzung des Geburtsgewichtes. Insgesamt wurden die besten Ergebnisse mit der Sabbagha-Formel erreicht (MPE 1.7 %; SD 9.0 %; MAPE: 6.0).

Schlussfolgerung: Mit der Sabbagha-Formel ist eine präzise Gewichtsschätzung in dieser Gruppe von Feten möglich. Es müssen jedoch die signifikanten Unterschiede bei der Gewichtsschätzung zwischen symmetrisch und asymmetrisch kleinen Feten beachtet werden.

Key words

ultrasound - sonographic - accuracy - weight estimation - SGA

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