Umbilical cord knot
False knots of the umbilical cord are relatively common. They characteristically result from either focal outpouchings or varicosities of the umbilical vessels or exaggerated looping of the umbilical blood vessels. False knots are of no clinical significance. The incidence of true knots in the umbilical cord ranges from 0.3 to 2.1% of all livebirths. Polyhydramnios, excessive fetal movement, and a long umbilical cord are considered predisposing factors to true knot formation. True knots have been associated with an increase in perinatal mortality. When the pathologic evaluation of a true knot in the umbilical cord reveals venous congestion, cord edema, and a local hematoma in the placental end of the cord, there has been sufficient vascular obstruction to result in fetal death.
The sonographic identification of true knots in the umbilical cord is limited to case reports. Collins and co-workers suggested that a ‘four-leaf clover’ pattern in the umbilical cord was diagnostic for a true knot. This pattern consists of two cord segments that cross and remain fixed in this position despite uterine manipulation. However, a ‘four-leaf clover’ pattern has also been identified in the presence of a false knot. Gembrach and Baschat have documented a stenotic effect on blood flow in the umbilical vein of a fetus with a true knot. Post-stenotic acceleration of venous flow occurred for approximately 2 cm distal to the stenosis. In addition, the umbilical venous flow velocity was pulsatile only in the post-stenotic region of the umbilical cord. Umbilical artery velocimetry of a tight knot in the umbilical cord reveals a systolic notch and reduced diastolic flow.
True umbilical cord knots must be distinguished from false knots. A false knot usually results from a varicosity of an umbilical vessel. False knots are not clinically significant. True knots of the umbilical cord have been associated with an increase in perinatal mortality.
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.