Ultrasonographic prediction of fetal macrosomia. Association with cesarean delivery

To investigate whether the incorrect ultrasonographic prediction of macrosomia affects the cesarean delivery rate among nonmacrosomic neonates.

For this retrospective, cohort study, comprehensive ultrasonographic records were reviewed at two centers. Patients with singleton, nonanomalous gestations whose ultrasonography predicted an estimated fetal weight > or = 4,000 g composed one cohort (n = 135), while the other cohort (n = 129) consisted of patients whose ultrasonography predicted an estimated fetal weight between 3,000 and 3,999 g. We compared the cesarean delivery rate in neonates falsely diagnosed with macrosomia (false positives) with the rate in those correctly diagnosed as nonmacrosomic (true negatives).

The rate of cesarean delivery was significantly higher among those falsely diagnosed by ultrasonography with a macrosomic fetus as compared to those with a fetus truly diagnosed as nonmacrosomic (42.3% vs. 24.3%, relative risk = 1.74, 95% confidence interval 1.09-2.78). Subgroup analyses excluding diabetic mothers and multiparous women and comparing false positives with true negatives with neonatal birth weights between 3,500 and 4,000 g (birth weights similar to false positives) demonstrated significantly increased cesarean delivery rates among false positives.

Even in nonmacrosomic neonates, the antenatal ultrasonographic diagnosis of suspected macrosomia is associated with a significant increase in cesarean delivery rates.