The prenatal diagnosis of a true umbilical cord knot is a challenging task during routine obsterical ultrasound. We propose the use of complementary three-dimensional (3D) HD Flow as a standardised technique to improve the diagnostic accuracy of ultrasound. The objective of this study is to identify if 3D-HD Doppler ultrasound can have a diagnostic role in the evaluation of a true umbilical knot. We conducted a prospective observational study in pregnant patients evaluated for second and third trimester ultrasound, from 2010 to 2021. Pregnancies with a single fetus and conventional ultrasound (gray scale, Doppler or Power Doppler), the suspicion of a true umbilical cord knot was included. This group of patients was further evaluated with the 3D-HD-Flow technique for its confirmation. This technique combines Doppler technology, with 3D umbilical cord reconstruction, for a more accurate visualisation of the umbilical cord morphology. The true umbilical cord knot was defined sonographically as an obvious entanglement of a segment of the umbilical cord diagnosed by this technique. All patients were followed until a postpartum umbilical cord evaluation was performed to confirm the initial diagnosis. During the study, approximately 9000 ultrasound studies of anatomical detail were performed, of which 17 reports (0.002%) suspected a true umbilical cord knot visualised with conventional ultrasound techniques. The mean gestational age at diagnosis was 30.1 weeks. Of the 17 cases included for the study, only 5 confirmed a true umbilical cord knot using the 3D-HD-Flow technique. All five cases had a long umbilical cord with a helical shape, 2 of them had intrauterine growth restriction, 3 babies developed transient tachypnea, and 2 babies had perinatal asphyxia. Of the 12 cases discarded with the 3D-HD-Flow technique, none had a real knot at the time of delivery. We suggest the use of the 3D-HD-Flow ultrasound technique as a complementary method to evaluate the true umbilical cord knot suspected in conventional ultrasound.