The umbilical cord connects the placenta to a developing baby, delivering oxygen and nutrients. Typically, it contains three blood vessels: one large vein, transporting oxygen-rich blood and nutrients to the fetus, and two smaller arteries, carrying deoxygenated blood and waste products away. However, in some pregnancies, the umbilical cord forms with only two vessels—a single vein and a single artery—a condition known as a single umbilical artery (SUA) or a two-vessel cord. This is the most common umbilical cord abnormality, affecting approximately 1 in 100 to 1 in 500 pregnancies.
Detection and Initial Implications
A single umbilical artery is most frequently identified during routine prenatal ultrasound examinations, particularly the anatomy scan around 18 to 22 weeks of gestation. Medical professionals look for the number of vessels in the umbilical cord, often observing them around the fetal bladder. A normal finding shows two arteries, while only one indicates SUA.
Upon detection, SUA is carefully considered. In about 75% of cases, it is an isolated finding, meaning the baby is otherwise healthy with no other associated medical issues. Nevertheless, its discovery prompts further evaluation. The cause for this developmental variation is not fully understood.
Associated Fetal Conditions
While SUA can be an isolated finding, it is also associated with a higher likelihood of other fetal conditions, warranting investigation. Chromosomal abnormalities, such as Trisomy 18 (Edwards Syndrome) and Trisomy 13 (Patau Syndrome), are more commonly linked with SUA, with Trisomy 21 (Down Syndrome) being a less frequent association. If other anomalies are present, the risk of chromosomal abnormalities can increase significantly.
Beyond chromosomal concerns, SUA is associated with an increased risk for congenital abnormalities, particularly those affecting the heart and kidneys. Cardiac anomalies, or congenital heart defects, are found in about 6-7% of babies with SUA, while renal (kidney) abnormalities, including malformations or the absence of a kidney, occur in approximately 5-6% of cases. Some kidney malformations might be subtle and not immediately apparent.
Less common, but possible, are gastrointestinal issues such as esophageal atresia or omphalocele. Fetal growth restriction (FGR), where the baby does not grow as expected, is also a recognized association with SUA, often necessitating careful monitoring of the baby’s size. The increased risk for these associated conditions is thought to stem from shared developmental pathways during early fetal formation.
Pregnancy Monitoring and Care
Once a single umbilical artery is diagnosed, a detailed monitoring and care strategy is implemented. Further ultrasound scans are usually recommended to check for any associated anomalies and to closely monitor the baby’s growth.
A fetal echocardiogram, a specialized ultrasound of the baby’s heart, is often advised to assess its function and structure, given the increased association with cardiac defects. Genetic counseling may also be offered to discuss the implications of SUA, especially if chromosomal abnormalities are suspected, and to review options for genetic testing, such as amniocentesis or non-invasive prenatal testing (NIPT).
Regular growth scans, particularly in the third trimester, track the baby’s size and well-being, as fetal growth restriction can occur. For isolated SUA, the mode or timing of delivery is generally not affected, but any identified associated conditions could influence delivery considerations.
Newborn Evaluation
Newborns diagnosed with a single umbilical artery undergo evaluation. A physical examination is performed immediately after delivery to check for any visible signs of abnormalities.
A postnatal kidney ultrasound is often recommended, even if prenatal scans appeared normal, to screen for renal abnormalities that might not have been evident before birth. Some kidney malformations can be subtle or develop postnatally. If any associated conditions were found during pregnancy or are suspected after birth, the baby will receive appropriate follow-up care, which may involve consultations with specialists and ongoing monitoring. For many babies with an isolated single umbilical artery, no further issues are identified, and they are expected to grow and develop without complications.