A 43-year-old woman underwent routine ultrasound examination at 35 + 5 weeks' gestation. An otherwise normal two-dimensional (2D) ultrasound examination (Voluson E8, GE Medical Systems, Zipf, Austria) revealed signs of a surrounding fixed loop (referred to as a 'hanging noose'1) in a transverse section of the umbilical cord (Figure 1). Axial section of umbilical cord knot on two-dimensional ultrasound. The umbilical cord was further examined using two- and three- (3D) dimensional imaging in conjunction with high-definition flow (HD-Flow™), which confirmed the presence of a true knot (Figures 2-4). Doppler sonography and fetal heart rate tracing were performed weekly until delivery. A Cesarean section was performed on maternal request at 39 weeks. A healthy male weighing 3090 g was delivered, with normal Apgar scores. A true knot was identified in the middle third of an 87-cm umbilical cord. Two-dimensional high-definition flow image of true umbilical cord knot. Three-dimensional high-definition flow image of true umbilical cord knot. Three-dimensional surface-rendering of true umbilical cord knot. We present here our observations on the usefulness of 3D-HD-Flow as a confirmatory technique in prenatal diagnosis of a true umbilical cord knot. Although they occur infrequently, true knots of the umbilical cord can have severe consequences. They occur in 0.04–3% of all deliveries, with associated perinatal morbidity in 11% of cases2, 3. The antepartum fetal mortality rate can be 4–10 times higher in such fetuses compared to the normal obstetric population, which makes prenatal diagnosis extremely valuable.2, 4-6 Other related perinatal complications have been described also, including oligohydramnios, intrauterine growth restriction and Cesarean section.4 Our case was not associated with any other perinatal complication. True knots have been reported in association with the following factors: advanced maternal age, history of previous miscarriage, maternal obesity, prolonged pregnancy, maternal anemia, umbilical cords measuring more than 80 cm, polyhydramnios, small fetus, male fetus, genetic amniocentesis, multiparous women, monoamniotic twins and other cord-related conditions such as prolapse. The positively associated findings in the case described here were advanced age, previous miscarriage, a male fetus and a long umbilical cord4, 5, 7-9. Prenatal diagnosis of a true umbilical cord knot is uncommon. The interposition of fetal parts, reduced amniotic fluid volume and difficulties in visualizing a complex three-dimensional structure on 2D ultrasound can make the diagnosis of a true cord knot challenging. We have described the use of 3D-HD-Flow as a complementary method to 2D ultrasound for prenatal diagnosis of an umbilical cord knot as it enabled its anatomical and functional characterization, thereby allowing the diagnosis to be made with greater confidence. Color Doppler is a well established method for prenatal diagnosis of fetal cord vascular alterations. The use of power Doppler and three-dimensional sonography has been previously described in the prenatal diagnosis of a true knot of the umbilical cord10. Complementary use of 3D-HD-Flow improves diagnosis as it is a more sensitive technique than power Doppler and also allows the direction of flow to be visualized. These characteristics allow the anatomic and vascular relationships of the cord loops to be determined. This method may also contribute to diagnosing cord entanglement in monoamniotic twins. However, its usefulness in the management of such pregnancies remains to be determined. The complementary use of 3D-HD-Flow is proposed in cases where true umbilical cord knots have been suspected after a 2D scan.