Prenatal diagnosis of true knot of the umbilical cord

The incidence of true knot of the umbilical cord has been reported to occur in 0.3–2.1% of all deliveries1 and the frequency of antepartum fetal death is four2, 3 to ten4 times higher in these fetuses. True knot of the umbilical cord often remains undiscovered prenatally because it is not associated with a characteristic clinical or in-utero ultrasound sign5. We describe the prenatal diagnosis of a true knot of the umbilical cord which we achieved in five cases using a new sonographic sign. During ultrasound examination of the umbilical cord, an unusual multicolored pattern was detected on color flow imaging. Detailed investigation showed a transverse section of the umbilical cord surrounded by a loop of umbilical cord (Figures 1 and 2). We referred to this image as a ‘hanging noose’ sign. The section of the umbilical cord was subjected to pressure from the surrounding knot (Figures 1 and 2). The examination ultimately revealed an image of a true knot of the umbilical cord. The pregnancies were monitored. The infants were delivered in good condition and prenatal findings were confirmed. Ultrasound images from convergent color Doppler examination at 38 weeks' gestation. (a) Hanging noose sign: transverse section of the umbilical cord surrounded by a loop of umbilical cord; the knot was not tight. (b) Pressure of the knot on the umbilical vein flow (arrow). A transverse section of the umbilical cord is visible over and under this portion of umbilical vein. (c,d,e) Images of the true knot of the umbilical cord, in which the knot is tighter. Ultrasound images at 33 weeks' gestation. (a) Power motion imaging examinations (a,c) and gray-scale image (b) of the hanging noose sign (arrow in (c)). (d) Power motion image showing pressure of the knot on the umbilical vein flow (arrow). Some sonographic characteristics, such as the four-leaf clover6, 7 or an unusual multicolored pattern visualized on color flow imaging4, have been described as being highly suspicious of true knot, but these findings have not been specific for the diagnosis5. The pattern described here may even be imaged in gray scale (Figure 2) but in color Doppler the view was clearer. The hanging noose sign along with the image of the section of umbilical cord under pressure from the knot are very suggestive of a true knot of the umbilical cord, particularly if these signs remain even after fetal movement.