A single umbilical artery can be seen in 1-2% of second and third trimester singleton births and up to 3.5% of twin pregnancies. It can be a normal isolated finding; however, if there are other anomalies present, the rate of aneuploidy increases to 50% (e.g. Trisomy 18 and 13). No association with trisomy 21 if it is an isolated finding.
In the majority of cases (70%), the left umbilical artery is absent. Intrauterine growth retardation can be seen in 15% and may be related to poor coiling. A higher risk of perinatal mortality is also noted. The SUA is larger than the normal umbilical artery, typically >50% diameter of the umbilical vein.
Other associated anomaliles most commonly occur in the musculoskeletal system (23%), genitourinary (20%), and cardiovascular (19%). If SUA is not an isolated finding, amniocentesis is indicated due to the higher rate of aneuploidy.
Fused umbilical arteries usually occur within 3 cm of the placenta and is not associated with aneuploidy. Umbilical vessel thrombosis is rare but is associated with maternal thrombophilia and also has a high fetal mortality. Excessive Wharton jelly causes an excessive gelatinous stroma and a thick umbilical cord with umbilical vessels displaced from each other. There is an association with Trisomy 21 and increased risk in the first trimester.