Three-dimensional power Doppler sonography in the prenatal diagnosis of a true knot of the umbilical cord: value and limitations.

OBJECTIVE: The purpose of this study was to examine the value of 3-dimensional power Doppler sonography in the prenatal diagnosis of a true knot of the umbilical cord. METHODS: Cases in which the diagnosis of a true knot of the umbilical cord was suspected by prenatal 2-dimensional sonography were reviewed. The presumably affected segment of the cord was examined with 3-dimensional power Doppler sonography for further characterization. Confirmation of the prenatal diagnosis was sought by reviewing the delivery records and contacting the referring obstetrician and the patients themselves. RESULTS: Eight consecutive cases were studied. Three-dimensional power Doppler sonography displayed a vascular spatial configuration pattern consistent with a true knot of the umbilical cord in all of them. However, the prenatal diagnosis was confirmed at delivery in only 5 cases (62.5%). Although there were no cases of a false knot mimicking a true knot of the umbilical cord, all incorrect diagnoses in this series were associated with multiple loops of the umbilical cord in the third trimester. CONCLUSIONS: Three-dimensional power Doppler sonography seems to be helpful in determining the presence of a true knot of the umbilical cord in utero, especially in the second trimester. However, this should not be considered a definitive method for the diagnosis because multiple loops of the umbilical cord lying close to each other can generate a sonographic image that can be undistinguishable from a true knot of the umbilical cord prenatally, especially when located in a small pocket of amniotic fluid. Therefore, the presumable diagnosis of a true knot of the umbilical cord in utero should be taken with caution.