What is the sensitivity and specificity of ultrasonography in identifying macrosomia in gestational diabetes mellitus (GDM)?

Updated: Apr 29, 2020
  • Author: Thomas R Moore, MD; Chief Editor: George T Griffing, MD  more...
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In a study involving more than 300 fetuses who weighed more than 4000 grams at birth, ultrasonography was found to have a sensitivity of only 65% in identifying macrosomia. However, a sensitivity of approximately 80% is typically associated with a specificity of 50-60%. This means a false-positive rate of 30-50% occurs even with the more predictive formula, possibly requiring an unnecessary cesarean delivery of more than 100 fetuses in order to prevent 1 from having permanent Erb palsy.

Thus, current data do not support a policy of early induction of labor in cases of possible fetal macrosomia. If one accepts that 8-20% of infants of diabetic mothers born weighing 4500 g or more will experience shoulder dystocia, 15-30% of these will have recognizable brachial plexus injury, and 5% of these injuries will result in permanent deficit, approximately 333-1667 cesarean deliveries would have to be performed for possible macrosomia to prevent 1 case of permanent injury due to shoulder dystocia. However, if fetal weight is estimated to be 4500 g or more, the risks and benefits of cesarean delivery should be discussed with the patient.

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