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It is important to ensure that the measurement is truly transverse and
at right angles to the long axis of the fetal spine (axial). Oblique
sections through the abdomen will result in measurement errors:
- Section A is a true axial
plane. The umbilical vein lies within the anterior third of the fetal
abdomen. Only a short segment is seen because the scan plane is axial
to the course of the vein
- Section B is through the
abdomen at the point at which the umbilical vein joins the portal
vein within the liver. Although this section can be used to measure
the abdomen, it may be difficult to differentiate from section C
unless the anatomy is clearly understood
- Section C shows the oblique
plane through the abdomen, following the course of the left portal
vein and the ductus venosus. This will produce inaccurate
measurement