EP08.39: The association between abnormal umbilical vein flow and undetected small for gestational age: a secondary analysis of a multicentre prospective study

To evaluate the correlation between umbilical vein flow (UVF) measured close to term with neonates born SGA that were not detected prenatally in a cohort of low-risk pregnancies. We hypothesise that undetected SGA is more common in fetuses with abnormal UVF than in fetuses with normal UVF. Secondary analysis of a prospective, multicentre study conducted across two tertiary Maternity units between April 2021 and March 2023. Uncomplicated singleton pregnancies between 35-38 weeks of gestation were included. Pregnancy at high-risk of placental insufficiency (EFW<10th percentile or abnormal Doppler) or with fetal anomalies were excluded. At US examination, the abdominal circumference (AC), UV diameter and peak velocity of the UV were measured. From these variables, we calculated the UVF/AC. The main outcome of the study was undetected SGA, defined as a birthweight below the 10th percentile according to gestational age. Overall, 379 women were included in the study, of whom 16 (4.2%) were born SGA. Undetected SGA was associated with a lower median UVF/AC (4.8 (IQR 3.9 – 6.6) ml/min/cm vs 6.1 (IQR 4.6 – 7.8) ml/min/cm; p = 0.02). The UVF/AC yielded an AUC of 0.65 (95% CI 0.53 – 0.77, p = 0.04) for the prediction of undetected SGA. The optimal cut-off value of the UVF/AC to predict fetal SGA was 4.8 ml/min/cm. This value was associated with a sensitivity of 0.56 (95% CI 0.30 – 0.80) and a specificity of 0.72 (95% CI 0.67 – 0.76), and a positive and negative likelihood ratio of 1.98 (95% CI 1.25 – 3.15) and 0.61 (95% CI 0.35 – 1.07); respectively. Moreover, while 3/5 (60.0%) cases of undetected SGA fetuses with an abnormal UVF/AC (below 20th percentile) suffered from stunted fetal growth, this occurred only in 2/9 (18.2%) with normal UVF/AC. Our results suggest that undetected SGA is associated with lower values of UVF/AC in a low-risk population. Moreover, if UVF/AC falls below the 20th percentile, fetuses are more likely to present concomitantly abnormal fetal growth patterns.