Umbilical Vein Doppler Evaluation in Small for Gestational Age Fetuses With Normal Umbilical Artery Doppler Findings: A Prospective Cohort Study.

The umbilical vein (UV) plays a crucial role in fetal oxygenation, and its dysfunction may contribute to adverse outcomes in small-for-gestational-age (SGA) fetuses. We hypothesized that UV Doppler assessment could enhance risk stratification and guide delivery timing. This study compared UV hemodynamics between SGA and appropriately-for-gestational-age (AGA) fetuses to evaluate their predictive value for low birth weight. In this prospective cohort study (Tehran, Iran; July-December 2024), we included singleton pregnancies >32 weeks. SGA was defined as estimated fetal weight (EFW) or abdominal circumference (AC) <10th percentile, excluding fetal growth restriction (EFW/AC <3rd percentile or additional Doppler abnormalities). UV flow (UVF), EFW-normalized UVF (nUVF), and UV diameters (intra-abdominal and free-loop) were compared between SGA and AGA cohorts. Predictive performance for low birth weight (<2500 g) was assessed via receiver operating characteristic (ROC) analysis. Among 322 pregnancies (78 SGA, 24.2%), SGA fetuses exhibited significantly lower UVF (172.1 versus 216.3 mL/min; P < .001), nUVF (76.4 versus 81.4 mL/minute/kg; P = .048), and narrower UV diameters (both P < .001). After adjustment, differences persisted except for nUVF. UVF and UV diameters moderately predicted low birth weight (areas under the ROC curve [AUCs] > .70), with free-loop diameter showing the highest discrimination (AUC = .798, 95% confidence interval: 0.687-0.909). SGA fetuses with normal umbilical artery Doppler demonstrate significant UV hemodynamic changes. UV Doppler, particularly free-loop diameter, may help identify high-risk pregnancies, aiding delivery planning. These findings support further exploration of UV assessment in SGA management protocols.