Sonographic Features
Color Doppler assessment of a suspected true knot reveals a multicolor pattern due to superimposed multidirectional pulsatile arterial and continuous venous flow at different velocities.
This pattern is more commonly seen with the cord entanglement associated with monoamniotic twins.
The narrowing of the arterial lumen with a true knot produces a rapid increase in resistance and a reduction in flow.
Arterial flow is reduced during diastole because of the high impedance produced by the stenosis.
In most cases a true knot in the umbilical cord is of little clinical significance.
However, sonographic detection should result in fetal surveillance with Doppler velocimetry and standard fetal biophysical assessment.