The Predictive Accuracy of Prenatal Ultrasound for Macrosomia—A Retrospective Cohort Study

OBJECTIVES: This study aimed to evaluate the accuracy of third-trimester ultrasound for fetal macrosomia and to assess obstetric outcomes associated with its prenatal suspicion. METHODS: Retrospective cohort study on women receiving prenatal care at a tertiary care hospital between November 2017 and December 2022. Singleton pregnancies with prenatal suspicion of macrosomia (estimated fetal weight ≥ 4000 g or > 90th percentile for gestational age) were included. Controls were singleton term deliveries without suspected macrosomia. Cases were categorized as true positives (TP) and false positives (FP), based on birthweight. Diagnostic accuracy for neonatal macrosomia was evaluated. Obstetric outcomes were compared between cases and controls, and between the TP and FP groups. RESULTS: A total of 365 patients received a prenatal suspicion of macrosomia during the study period, of whom 239 (65.48%) were classified as TP and 126 (34.52%) as FP. The FP rate did not differ significantly whether the ultrasound assessment was performed before or after 37 weeks. The prenatal suspicion of fetal macrosomia presented high specificity (98.49%) and negative predictive value (97.78%), but low sensitivity (63.75%) and positive predictive value (72.34%) for neonatal macrosomia. Additionally, a prenatal suspicion of macrosomia was strongly associated with labor induction (57.26% vs. 25.52% in the control group, p < 0.001), with 27.27% of inductions occurring before 39 weeks. CONCLUSIONS: Third-trimester fetal weight estimation has limited predictive accuracy for neonatal macrosomia, and sonographic suspicion of macrosomia is strongly associated with labor induction. These findings suggest that suspected prenatal macrosomia should not be used in isolation when planning obstetric management, including decisions regarding labor induction.