Routine third-trimester ultrasound screenings for large for gestational age (LGA) are associated with increased intervention with no clear benefit, according to new research published in the American Journal of Obstetrics & Gynecology.

In a retrospective cohort study of more than 21,700 singleton pregnancies, investigators found that screening at 35-37 weeks had limited ability to accurately identify LGA infants, with a sensitivity of just under 35%. Roughly 1 in 3 pregnancies labeled as suspected LGA were false positives, yet the diagnosis was associated with a 66% higher risk for cesarean delivery and a 45% higher risk for perinatal complications including maternal outcomes such as postpartum hemorrhage and neonatal outcomes such as NICU admission.

A positive LGA screen was defined as an estimated fetal weight at or above the 90th centile and excluded pregnancies with major fetal anomalies, genetic abnormalities, or missing outcome data. The study took place between 2019 and 2025 at a hospital in London, England.

Investigators found a “labeling effect,” in which pregnancies flagged as suspected LGA were managed more aggressively regardless of the size of the fetus. Cases labeled as such were 13% less likely to proceed with a trial of labor and 22% less likely to have a vaginal delivery.

“A diagnostic label can shape perception of risk,” said Asma Khalil, MD, a consultant in maternal fetal medicine at City St George’s, University of London in London, who led the study. “If a fetus is thought to be large, clinicians may anticipate complications such as shoulder dystocia or labor dystocia, and patients may also feel more anxious about vaginal birth. That can influence counseling, shared decision-making, and thresholds for intervention.”

Cases where fetuses were incorrectly identified as suspected LGA were associated with increases in cesarean delivery, operative vaginal delivery, and maternal complications, with no improvement in neonatal outcomes such as hypoglycemia and seizures.

Missed LGA cases were associated with more favorable outcomes, with lower rates of cesarean delivery, labor induction, and maternal complications, without an increase in neonatal complications.

“These findings suggest that clinical behavior may be influenced as much by perception of risk as by true risk,” Khalil said.

The study aligns with earlier research demonstrating the limitations of third-trimester LGA screens. Previous studies have suggested that a prenatal diagnosis of suspected LGA may increase intervention rates.

The findings raise important questions about the clinical value of universal LGA screening late in pregnancy, according to Xiao Wang, MD, an assistant professor of obstetrics and gynecology at Northwestern University Feinberg School of Medicine in Chicago, who was not associated with the study.

“More information isn’t always better,” Wang said. “Sometimes more information in an otherwise low-risk group may actually lead to worse outcomes.”

If a low-risk patient receives an LGA diagnosis, clinicians should be cautious not to overreact, Wang said.

Khalil emphasized the need for more individualized, risk-based approaches that incorporate the broader clinical context.

“Patients should understand that ultrasound-estimated fetal weight is helpful but not exact and that there is a margin of error,” Khalil said. “The discussion should include the woman’s overall clinical picture, such as diabetes status, prior births, maternal BMI, gestational age, and preferences, rather than relying on fetal size alone.”

The sources cited in the article reported having no disclosures. No funding was reported.

Brittany Vargas is a journalist covering medicine, mental health, and wellness.