True umbilical cord knot

True umbilical cord knots are a rare occurrence and as the name suggests represent a knot formation in the umbilical cord.

Epidemiology

They occur in less than 1% of pregnancies.

Pathology

Predisposing factors

• a long umbilical cord
• polyhydramnios
• small fetus
• excessive fetal movements.

Radiographic features

Ultrasound

• focal dilatation of umbilical vein or umbilical vein thrombosis near the umbilical cord knot- giving a false appearance of an umbilical cord cyst.
• tortuosity of cord at the level of knot
• a segment of the umbilical cord that is closely surrounded by another loop of umbilical cord is considered to be highly specific of a true knot and is known as hanging noose sign

Prognosis

Most of the knots formed in the umbilical cord are loose and thereby do not adversely impact the fetal well being; however, due to fetal movement or descent, the knot could get tighter, thereby decreasing the umbilical cord blood flow. This could further lead to fetal asphyxia and demise.

See also

• false knot
• umbilical cord knot