Umbilical Cord Abormalities

The normal umbilical cord is a spiral of two umbilical arteries and one umbilical vein. The umbilical arteries originate from the fetal internal iliac arteries and pass on either side of the fetal urinary bladder. The umbilical vein drains into the fetal hepatic vein.

Two-Vessel Umbilical Cord

The normal umbilical cord has two arteries and one vein. A two-vessel cord has a single umbilical artery, is seen in up to 1% of pregnancies, and is more common in multiple gestations (especially monozygotics) and in diabetic mothers.

When suspected, the entire umbilical cord must be inspected to look for the second umbilical artery. If the second umbilical artery is seen in any part of the umbilical cord, then it is considered a three-vessel cord.

A two-vessel cord is important because about 30% of fetuses with will have structural anomalies, most commonly cardiovascular and renal. In addition, infants with a two-vessel cord have lower birth weights and higher perinatal mortality rates. Isolated cases of a two-vessel cord are not associated with a risk of aneuploidy.

On prenatal ultrasound, one sees the larger umbilical vein, with flow toward the fetus and the smaller umbilical artery, with flow away from the fetus. Imaging of the fetal pelvis will show an umbilical artery only on one side of the urinary bladder.

The finding of a single umbilical artery should prompt a detailed evaluation of the fetus with special emphasis on the heart and kidneys. If no other anomaly is found, clinical evaluation and/or follow-up ultrasound is recommended to evaluate fetal growth, since there is an association with low birth weight.

Umbilical Knots

Umbilical knots may be true knots or false knots. True knots, which are seen in less than 1% of pregnancies, occur more frequently in long cords, male fetuses, and multiparous women and are associated with a perinatal mortality rate of about 10% . Loose knots early in pregnancy may tighten with fetal movement or during delivery.

Parents should be counseled about the increased risk of intrauterine fetal demise and the pregnancy closely monitored with umbilical artery Doppler until term. Continuous cardiotocography during labor is recommended.

A true knot presents as a localized distention of the umbilical vein on prenatal ultrasound. One side of the distention can be traced into the umbilical vein, while the other side terminates abruptly in the knot. In addition, the vein is very difficult to trace.

True knots can have an appearance similar to small allantoic or omphalomesenteric cysts (see below). In these cases, continuity with the umbilical vein on one side confirms the diagnosis of a knot.

True knots may also have an appearance similar to the rare extra-abdominal umbilical vein varix (see below). An extra-abdominal umbilical vein varix appears as tortuous fusiform dilatations that is in smooth continuity with the umbilical vein on both sides, in contrast to the blind-ending true knot.

False knots are not considered problematic. The term may refer either to focal umbilical venous outpouchings, more appropriately called varices (see below), or exaggerated looping of umbilical cord vessels within the cord.

Umbilical Cord Varices

An umbilical venous varix is a focal dilation of the umbilical vein. It rarely occurs outside the fetus, and is most commonly found in the intra-abdominal portion of the umbilical vein between the anterior abdominal wall and the liver.

Intra-abdominal umbilical vein varices may be associated with fetal abnormalities, such as fetal anemia, and may be an early sign of hydrops. Rarely, a varix of the umbilical vein will be present within the umbilical cord outside the fetus.

On ultrasound, we see focal, usually saccular, dilation of the umbilical vein, just deep to the umbilical cord insertion oriented in the anteroposterior direction at the level of the liver. More fusiform varices are rarely seen and can extend into the intrahepatic portion of the umbilical vein.

An extra-abdominal umbilical vein varix appears as tortuous fusiform dilatations that is in smooth continuity with the umbilical vein on both sides.

Differential considerations for intra-abdominal varices include choledochal cysts (oriented in a craniocaudal direction), hepatic cysts (rare, inside the liver), urachal cysts (extend toward the pelvis), or mesenteric cysts (more rounded and lower in the abdomen).

Allantoic Duct Cyst

Allantoic duct cysts are seen within the umbilical cord and may be seen in association with fetal anomalies (e.g., omphalocele and aneuploidy).

Prenatal ultrasound shows a cyst within the umbilical cord adjacent to the umbilical vessels.

Differential considerations include true knots of the cord (see above) and focal areas of excess Wharton's jelly. The latter are easily differentiated by their echogenicity and lack of well-defined thin walls.

References

• Hasbun J, Alcalde JL, Sepulveda W. Three-dimensional power Doppler sonography in the prenatal diagnosis of a true knot of the umbilical cord: value and limitations. J Ultrasound Med. 2007 Sep;26(9):1215-20.
• Hertzberg BS, Bowie JD, Bradford WD, Bolick D. False knot of the umbilical cord: sonographic appearance and differential diagnosis. J Clin Ultrasound. 1988 Oct;16(8):599-602.
• Jeanty P. Fetal and funicular vascular anomalies: identification with prenatal US. Radiology. 1989 Nov;173(2):367-70.
• Scioscia M, Fornalè M, Bruni F, Peretti D, Trivella G. Four-dimensional and Doppler sonography in the diagnosis and surveillance of a true cord knot. J Clin Ultrasound. 2011 Mar;39(3):157-9.
• Van den Hof MC, Wilson RD; Diagnostic Imaging Committee, Society of Obstetricians and Gynaecologists of Canada; Genetics Committee, Society of Obstetricians and Gynaecologists of Canada. Fetal soft markers in obstetric ultrasound. J Obstet Gynaecol Can. 2005 Jun;27(6):592-636.