Discordance between abdominal circumference and estimated fetal weight in the late third trimester and risk of adverse perinatal outcomes.

BACKGROUND: Third-trimester fetal ultrasound demonstrating abdominal circumference (AC) and estimated fetal weight (EFW) ≥90th percentile is associated with delivery of a large for gestational age (LGA) newborn; however, the clinical significance of discordance between these measures is unclear. OBJECTIVE: To evaluate the association between discordant AC and EFW percentiles and adverse perinatal outcomes. STUDY DESIGN: We conducted a retrospective cohort study of singleton pregnancies with a growth ultrasound performed between 32w0d and 36w6d at a single academic center (2017-2024). Fetuses were categorized into four groups based on 90th percentile thresholds for EFW and AC: (1) both <90th percentile (reference), (2) isolated large EFW (≥90th, AC<90%), (3) isolated large AC (≥90th, EFW<90%), and (4) both ≥90th percentile. The primary outcome was large-for-gestational age (LGA) birth (birthweight ≥90% by Fenton curve). Secondary outcomes included macrosomia (birthweight ≥4000g), primary cesarean delivery, and shoulder dystocia. Using marginal standardization (parametric G-formula), we estimated adjusted risks, relative risks (aRR) and risk differences (aRD) controlling for maternal age, BMI, parity, and diabetes status. RESULTS: Among 13,592 eligible pregnancies, 75.7% had both EFW and AC<90th, 9.7% had isolated large EFW or AC, and 14.6% had both ≥90th percentile. Overall, 12.6% of neonates were LGA. Compared with the reference group, adjusted risk of LGA rose to 17.9% for isolated large AC (aRR 4.3, 95%CI 3.6-5.1), 21.5% for isolated large EFW (aRR 5.2, 95%CI 4.1-6.5), and 48.2% when both AC and EFW were ≥90th percentile (aRR 11.4, 95%CI 10.2-12.8). Similar stepwise trends were observed for macrosomia, primary cesarean delivery, and shoulder dystocia. Associations were consistent across sensitivity analyses, including stratification by diabetes status. CONCLUSIONS: Compared to both AC and EFW <90th percentile, isolated third-trimester AC or EFW ≥90th percentile is associated with adverse perinatal outcomes. Patients with an isolated large AC should be counseled regarding these elevated perinatal risks.