Intra-amniotic umbilical cord vein velocity: is it useful for identifying small for gestational age fetuses?

Research has shown that umbilical vein velocity is persistently lower in growth restricted fetuses. This conclusion was evident in both longitudinal and cross-sectional studies, in different sample sizes, in both intra-abdominal and intra-amniotic sampling sites and across a range of gestational ages. However, is this reduced velocity large enough to distinguish between birthweight categories? Linear mixed modelling showed a significant difference between the UCVPV of the three birthweight categories (p < 0.001) and each category had different shaped curves (p = 0.002). However, the confidence intervals of the birthweight category regression curves overlapped between 20 weeks and 1 day and 35 weeks and 1 day GA. ICC demonstrated excellent agreement within (0.99) and between sonographers (0.98). A curvilinear relationship between umbilical vein velocity and GA has been reported previous. The confidence intervals of the birthweight category regression curves overlapped during a 15 week period of crucial fetal growth. Differences between categories were not identifiable during this period inferring that UCVPV could not reliably isolate a SGA fetus.