True knot of the umbilical cord: transient constrictive effect to umbilical venous blood flow demonstrated by Doppler sonography.

True knot of the umbilical cord is a rare occurrence, which is difficult to diagnose antenatally, even with the use of ultrasound. Constriction of a true knot of the umbilical cord may lead to obstruction of the fetal circulation and subsequent intrauterine death. We present a singleton pregnancy, in which the antenatal diagnosis of a true knot of the umbilical cord with entanglement of the cord around the fetal neck and body was made at 23 + 1 weeks' gestation. With the use of color-coded Doppler followed by pulsed wave Doppler spectral analysis, a stenotic effect on the umbilical venous blood flow, with normal arterial blood flow, was demonstrated for the first time. Marked acceleration of umbilical venous blood flow velocities from 15 cm/s pre-stenotically to 100 cm/s post-stenotically was demonstrated. This presented as an aliasing phenomenon and a mosaic pattern on color flow imaging in the variance mode. This phenomenon was only transient and could not be demonstrated again at follow-up examinations. A healthy newborn was delivered by Cesarean section at 33 + 1 weeks' gestation. The presence of a single knot was confirmed at delivery.