Objective

To compare delivery outcomes between true positive (TP) and false positive (FP) LGA, appropriate for gestational age (AGA) and false negative LGA (FN LGA) fetuses.

Methods

Retrospective cohort study of singleton pregnancies at risk for macrosomia without contraindication to vaginal delivery, receiving an ultrasound scan at 34-37 weeks.

Results

430 pregnancies were included: 155 TP LGA, 87 FP LGA and 177 AGA and 11 FN LGA newborns. Cesarean section rate during labour was significantly higher in FP LGA than in AGA (19% vs 8.7%) but not significantly different between FP LGA and TP LGA (19% vs 32.4%). Median birth weight z score was significantly higher in TP LGA (1.9) compared to the FP LGA and AGA (0.91 and 0.84, respectively), while no significant differences were found between FP LGA and AGA. Admission to neonatal intensive care unit was significantly more frequent in TP LGA than AGA, while shoulder dystocia, postpartum hemorrhage, and 3rd-4th degree perineal tears were similar between the different groups.

Conclusion

A false positive diagnosis of LGA fetus is associated with a significant increase of Cesarean section during labour. Therefore, a suspicious ultrasound may result in reduction of the clinical threshold for the diagnosis of abnormal labour.