SCREENING FOR LARGE-FOR-GESTATIONAL-AGE NEONATES AT TERM: EVIDENCE OF A LABELLING EFFECT AND INCREASED INTERVENTION WITHOUT NEONATAL BENEFIT

OBJECTIVE: To evaluate the diagnostic accuracy of routine third-trimester ultrasound for detecting large-for-gestational-age (LGA) and macrosomic neonates at term, and the impact of inaccurate prenatal diagnosis on obstetric management and perinatal outcomes. STUDY DESIGN: Retrospective cohort study of 21,743 singleton pregnancies undergoing routine 35-37 weeks ultrasound at a tertiary referral center. Pregnancies with major fetal anomalies, genetic abnormalities, or missing outcome data were excluded. Estimated fetal weight ≥90th centile defined screen-positive status; birthweight ≥90th centile and >4000g defined LGA and macrosomia at birth, respectively. Study outcomes included mode of delivery, and composite adverse perinatal outcomes comprising maternal and neonatal outcomes. Composite adverse maternal outcome included postpartum hemorrhage, high dependency or intensive care unit admission, obstetric anal sphincter injury and intrapartum cesarean delivery, while composite adverse neonatal outcome included neonatal intensive care unit admission >48 hours, continuous positive airway pressure support, mechanical ventilation, hypoxic-ischemic encephalopathy, intraventricular hemorrhage, respiratory distress syndrome, seizures, hypoglycemia, transient tachypnea of the newborn, meconium aspiration syndrome and death. Adjusted relative risks (aRR) with 95% confidence intervals were estimated using modified Poisson regression, adjusting for maternal and pregnancy characteristics. Diagnostic performance measures, including sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV), positive (LR+) and negative (LR-) likelihood ratios and 95% confidence intervals, were calculated. The interval between the index test and reference standard was the time from ultrasound to delivery. Clinicians were aware of ultrasound findings, which informed counselling and management. Results are reported as adjusted relative risks (aRR) with 95% confidence intervals (CI). RESULTS: Ultrasound screening showed a sensitivity of 34.9% (33.2-36.6) for LGA and 35.6% (33.5-37.9) for macrosomia, with specificity of 97.4% (97.2-97.6) and 95.6% (95.3-95.9), respectively. Screen-positive pregnancies were significantly less likely to attempt labor (aRR 0.87, 95% CI [0.84-0.90]; p<0.001) and more likely to undergo intrapartum cesarean delivery (aRR 1.47, 95% CI [1.30-1.67]; p<0.001), with higher risks of composite adverse maternal (aRR 1.43, 95% CI [1.32-1.55]; p<0.001) and neonatal (aRR 2.37, 95% CI [1.85-3.05]; p<0.001) outcomes. Compared to true-positive cases, false-negative cases were associated with significantly lower rates of induction of labor (aRR 0.74, 95% CI [0.68-0.81]; p<0.001), intrapartum cesarean delivery (aRR 0.70, 95% CI [0.59 - 0.83]; p<0.001) and composite adverse maternal outcomes (aRR 0.87, 95% CI [0.77-0.98]; p<0.032), with no increased risk of adverse neonatal outcomes (aRR 0.77, 95% CI [0.56-1.06]; p=0.12). Compared to true-negative cases, false-positive cases experienced higher rates of operative vaginal delivery (aRR 1.29, 95% CI [1.07-1.57]; p<0.001), intrapartum cesarean delivery (aRR 1.25, 95% CI [1.0-1.55]; p=0.045) and composite adverse maternal outcomes (aRR 1.28, 95% CI [1.11-1.48]; p<0.001) with no difference in the risk of adverse neonatal outcomes (aRR 1.35, 95% CI [0.72 - 2.53] p=0.347) CONCLUSION: Routine third-trimester ultrasound screening for suspected LGA and macrosomia demonstrates limited diagnostic performance and appears to generate a labelling effect; whereby obstetric intervention and some adverse outcomes may be influenced more by suspected LGA status than by fetal size itself. These findings raise questions about the benefit of universal LGA screening at term and support consideration of more individualized, risk-based approaches to late-pregnancy assessment.