Three‐dimensional ultrasound with color Doppler imaging of an umbilical cord true knot

True knot of the umbilical cord is relatively rare, being found in 0.3–2.1% of all births1. It is associated with a four-fold increase in fetal death rate, an increase in non-reassuring fetal heart rate pattern in labor and Cesarean delivery1. Obstetric factors that are significantly correlated with a true knot of the umbilical cord include long umbilical cord, small fetus, polyhydramnios, gestational diabetes, male fetus and genetic amniocentesis1. Prenatal diagnosis of an umbilical cord true knot is possible with modern ultrasound equipment2. The presence on gray-scale ultrasound of a cross-section of an umbilical cord surrounded by a circular cord loop should raise suspicion for the presence of a true knot. Confirmation can be achieved by acquiring a color Doppler three-dimensional volume of the suspected anatomical section of the cord and rotating the volume along the x- and y-axes until a full view of the knot is displayed (Figure 1). In a cord with a true knot, pulsed Doppler velocimetry of the umbilica vessels may display abnormal findings which have been correlated with cord compression in a constricted knot. A stenotic effect on the umbilical venous blood flow was demonstrated antenatally in a 23+1-week fetus with a true umbilical cord knot3. Furthermore, the presence of notching in the umbilical artery waveform has been reported in fetuses with true umbilical cord knots and correlated with cord compression4. Although the impact on neonatal outcome of a prenatal diagnosis of a true knot or entanglement in the umbilical cord has not been fully evaluated, a recent publication noted that cord entanglement does not contribute to prenatal morbidity and mortality in monoamniotic twin pregnancies5. Until further evidence emerges, we suggest that the diagnosis of an umbilical cord true knot warrants careful evaluation of the umbilical artery blood flow pattern and close monitoring of pregnancy and delivery.