(542) Third-Trimester Biometric Discordance Between AC and EFW and Risk of Adverse Outcomes

We assembled a retrospective cohort of singleton pregnancies with a growth ultrasound performed between 2017 and 2024 at a single academic center. We restricted the analysis to the first scan performed between 32w0d and 36w6d. EFW and AC percentiles were calculated using Hadlock standards. Fetuses were stratified into four groups: (1) EFW< 90%/AC< 90%, (2) EFW≥90%/AC< 90%, (3) EFW< 90%/AC≥90%, and (4) EFW≥90%/AC≥90%. Outcomes included LGA (birthweight≥90% by Fenton), macrosomia (≥4000g), primary cesarean, and shoulder dystocia. Multivariable models estimated adjusted relative risks, controlling for maternal age, BMI, parity, and diabetes (pregestational and gestational), using the EFW< 90%/AC< 90% group as the reference.

Results:

Among 12,781 pregnancies, 10% (n=1,263) demonstrated discordance between EFW and AC percentiles (only AC or EFW≥90%), while 15% (n=1,863) had both≥90%. Discordant biometry was associated with a 4-5-fold increased risk of LGA and macrosomia and modestly increased risk of primary cesarean and shoulder dystocia. Notably, isolated AC≥90% conferred a >2-fold higher shoulder dystocia risk, even in the absence of large EFW. Concordant elevation (EFW and AC≥90%) was associated with >10-fold higher risk of LGA and 5-fold higher risk of shoulder dystocia. These patterns were consistent among both diabetic and non-diabetic patients.

Conclusion:
Third-trimester discordance between AC and EFW≥90% is associated with increased risk of adverse perinatal outcomes. While discordance occurred in only 10% of pregnancies, it resulted in more than twice the rate of LGA and shoulder dystocia compared to the concordant normal group. These associations held true regardless of glycemic status. These findings support incorporating AC% into third-trimester counseling, even when EFW alone does not meet the≥90% threshold.