Routine 3rd trimester ultrasound for large-for-gestational-age shows limited performance

A retrospective cohort study published April 13, 2026, in The American Journal of Obstetrics & Gynecology (AJOG) found that routine 3rd-trimester ultrasound screening for suspected large-for-gestational-age (LGA) and macrosomic neonates demonstrated limited diagnostic performance. The research suggested that the detection of a suspected large fetus may trigger a labeling effect, where obstetric interventions and certain adverse outcomes are influenced more by the prenatal diagnosis than by the actual birthweight of the infant.

The study evaluated 21,743 singleton pregnancies undergoing routine ultrasound at 35 to 37 weeks of gestation at a tertiary referral center. Researchers defined screen-positive status as an estimated fetal weight at or above the 90th centile. At birth, LGA was defined as a birthweight at or above the 90th centile, whereas macrosomia was defined as a birthweight greater than 4000g. Pregnancies involving major fetal anomalies or genetic abnormalities were excluded from the analysis to focus on routine screening outcomes in a standard population.

Diagnosing LGA and screening performance

The results indicated that ultrasound screening had a sensitivity of only 34.9% (33.2 – 36.6) for detecting LGA and 35.6% (33.5 – 37.9) for macrosomia. Although the sensitivity was low, the specificity was high at 97.4% (97.2 – 97.6) for LGA and 95.6% (95.3 – 95.9) for macrosomia.

The interval between the ultrasound and delivery served as the time frame for the reference standard. Because clinicians were aware of the ultrasound findings, the results informed subsequent counseling and obstetric management for the participants.

Impact on obstetric management and outcomes

Screen-positive pregnancies, or those where a large infant was suspected, were significantly less likely to attempt labor (aRR 0.87, 95% CI [0.84–0.90]; P < .001) and more likely to undergo intrapartum cesarean delivery (aRR 1.47, 95% CI [1.30–1.67]; P < .001). These pregnancies also faced higher risks of composite adverse maternal outcomes (aRR 1.43, 95% CI [1.32–1.55]; P < .001) and composite adverse neonatal outcomes (aRR 2.37, 95% CI [1.85–3.05]; P < .001).

The composite adverse maternal outcome included factors such as postpartum hemorrhage, intensive care unit admission, obstetric anal sphincter injury, and intrapartum cesarean delivery. The composite adverse neonatal outcome included intensive care unit admission for more than 48 hours, respiratory distress, and hypoglycemia, among other serious complications.

Labeling effect of inaccurate diagnosis

Compared to true-positive cases (where a large fetus was correctly identified), false-negative cases (where a large fetus was missed) were associated with significantly lower rates of labor induction (aRR 0.74, 95% CI [0.68-0.81]; P < .001) and intrapartum cesarean delivery (aRR 0.70, 95% CI [0.59–0.83]; P < .001). Furthermore, false-negative cases had lower rates of adverse maternal outcomes (aRR 0.87, 95% CI [0.77–0.98]; P < .032) without increased risk to the neonate (P = .12).

Conversely, false-positive cases—where an infant was suspected to be large but was born at a normal weight—experienced higher rates of operative vaginal delivery (aRR 1.29, 95% CI [1.07-1.57]; P < .001) and intrapartum cesarean delivery (aRR 1.25, 95% CI [1.0-1.55]; P = .045) compared to true-negative cases. These false-positive pregnancies also saw a higher risk of adverse maternal outcomes (aRR 1.28, 95% CI [1.11–1.48]; P < .001) despite no difference in neonatal risk (P = .347).

“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,” the study investigators concluded.

Reference:

Lopian M, Ulusoy CO, Mohamed D, Segal E, Dhalil A. Screening for large-for-gestational-age at term: Evidence of a labelling effect and increased intervention without neonatal benefit. American Journal of Obstetrics & Gynecology. Published online April 13, 2026. Accessed April 14, 2026. doi:10.1016/j.ajog.2026.04.015