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Single Umbilical Artery
aka Two Vessel Cord

What is SUA? SUA is when the umbilical cord has only one artery, with the left artery absent more commonly than the right. Even though only one artery is necessary to sustain a pregnancy, single umbilical artery still causes concern because of other conditions that are often associated with SUA pregnancies. Single umbilical arteries are associated more commonly with fetal anomalies than normal cords.

How many arteries does a normal umbilical cord have? The umbilical cord is composed of three vessels; 2 arteries and one vein. Only 1 artery is necessary to sustain a healthy pregnancy. The vein carries oxygen-rich blood and nutrients from the placenta to the baby, while the two arteries transport waste from the baby back to the placenta (where waste is transferred to the mother’s blood and disposed of by her kidneys).

What causes only one artery? Single umbilical artery is believed to be caused by atrophy of a previously normal artery, presence of the original artery of the body stalk, or agenesis of one of the umbilical arteries.

How common is SUA? Single umbilical artery (SUA) is the most common malformation of the umbilical cord and affects between 0.5% and 7% of pregnancies and 1 in 100 live births. Single umbilical artery occurs in fewer than 1% of cords in singletons and 5% of cords in at least one twin. SUA's are found twice as often in white women than in African American and Japanese women. Diabetes increases the risk significantly. Two-vessel cords are found more frequently in fetuses aborted spontaneously. The male-to-female ratio is 0.85:1.

How is SUA diagnosed? The vessels in the cord are clearly identifiable with ultrasound. The vein usually is larger than the arteries. The presence of only 2 vessels on an ultrasound cross section is highly suggestive of a single umbilical artery.

Will having an SUA affect my pregnancy? There is some disagreement on the significance of this phenomenon; however, there is an association with low birthweight (<2500g) and early delivery (<37w)(20% of SUA pregnancies). The only change to antenatal care one might expect is a growth scan to make sure the baby is growing at a 'normal' rate toward the last month of pregnancy; with only a single artery to support the now large baby, upto 15-30% of the fetuses with SUA are at risk of intrauterine growth retardation.

Will SUA affect my baby? According to one study (see above chart), about a third of SUA affected babies had some other anomaly. Of that 1/3, half of the babies had structural heart abnormalities; slightly more than a third had some gastrointestinal tract abnormality; and one fourth had some malformation of the central nervous system. About 10-15% of babies with SUA have a chromosome (gene) abnormality, however the vast majority of these are picked up by ultrasound antenatally. The overall risk increase for an "abnormal" baby is around 4 times. Ultrasound scanning is very good at picking up other abnormalities if SUA is present, so a normal ultrasound is very reassuring.

What are the most likely additional problems that a baby with SUA might have? Of infants with a single umbilical artery, 20-50% have associated fetal anomalies, including cleft lip, cardiovascular abnormalities (especially ventricular septal defects and conotruncal defects), ventral wall defects, central nervous system defects, esophageal atresia, spina bifida, diaphragmatic hernia, cystic hygroma, hydronephrosis, dysplastic kidneys, polydactyly, syndactyly, and marginal and velamentous insertion of the cord. Of baby's with SUA, 30% have congenital anomalies, which may include trisomy 13, trisomy 18, and triploidy; trisomy 21 is uncommon.

Can my baby die from having a single umbilical artery? With single umbilical arteries, a 20% perinatal mortality rate exists. Two thirds of deaths occur before birth. Of the one third of neonates who die postnatally, most have associated congenital abnormalities. Many of those without overt abnormalities are premature or have intrauterine growth retardation.

What increases my risks for having SUA?

• Female fetal sex
• Maternal age over 40years
• 3 previous children
• Multiple fetuses

So what does all of this mean? SUA alone is of very little significance to the pregnancy. The main concern is of problems that SUA babies usually have in addition to having a two vessel cord. Some of the most common problems SUA babies have are related to the heart and kidneys/bladder, but these problems can, generally, be diagnosed via ultrasound. According to Ross Welch, "It is important to check with colour flow (I find colour power angio easiest for this) whether there is a visible hypogastric artery either side of the fetal bladder or whether there is only one. There are a couple of papers in the literature which report anastomosis of two vessels inside the fetus into one in the cord."

Resources:
Fetal Medicine Unit: Single Umbilical Artery. (Diagnosis ultrasound images.)
Dr. Hull's brief SUA article. (Dr. Hull's son had an SUA and was born healthy.)
March of Dimes: Umbilical Cord Abnormalities.

Support:
UCB Parents : Notes from moms whose pregnancies had a SUA.
Visit our High Risk Pregnancy or Pregnancy Forums for Support.