Single Umbilical Artery - Causes, Diagnosis, and Management

What Is Single Umbilical Artery (SUA)?

It is a condition in which the umbilical cord contains only one artery. Babies with SUA are at higher risk of congenital and chromosomal anomalies. The congenital anomalies can be musculoskeletal, cardiovascular, or renal.

A normal umbilical cord contains two umbilical arteries and an umbilical vein; the former transports oxygen-poor blood from the fetus to the placenta for metabolism, while the latter transports oxygen-rich blood from the mother to the fetus through the placenta.

SUA occurs in nearly 1 % of total pregnancies, with a higher prevalence in multiple pregnancies than in singleton pregnancies. It can be diagnosed with an ultrasound; however, it is best to determine the condition after the baby is born.

What Causes Single Umbilical Artery?

The umbilical cord is formed between 13 and 38 days after conception, and any abnormality can be observed as early as 13 weeks. Although the exact cause is unknown, there are three theories to explain how an SUA may form during development. They are as follows:

1. The first theory suggests agenesis (absence) of the one umbilical artery.

2. The second theory suggests atrophy or atresia of a previously normal umbilical artery.

3. The third theory describes a persistent allantoic artery as an explanation for SUA.

Embryological studies have observed occluded remnants of the second umbilical artery suggesting the second theory to be the most likely explanation for SUA.

What Are The Predisposing Factors for Single Umbilical Artery?

The following factors can increase the likelihood of SUA:

• Advanced maternal age.

• History of multiple births and multiple pregnancies.

• Gender (female babies fetus has higher chances of developing SUA).

• Smoking.

• Medical conditions like diabetes, high blood pressure, epilepsy, etc.

• Oligohydramnios and polyhydramnios of which are abnormalities of the amniotic fluid.

• Assisted reproductive technology has also been known to cause SUA.

The predisposing factors mentioned above have a systemic effect leading to other medical conditions along with SUA.

What Are the Associated Conditions Seen Along With SUA?

SUA is commonly associated with at least one of the following conditions:

1. Monozygotic Twins - A condition that results from the fertilization of a single egg by a single sperm, with the fertilized egg then splitting into two; the twins formed are identical in nature.

2. Sirenomelia - It is a rare congenital deformity in which the legs are fused, giving them the appearance of the tail of a mermaid. The condition is caused due to the lack of an enzyme that degrades retinoic acid.

3. VACTERL Association - It is a syndrome that comprises vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies, and limb abnormalities. The condition is caused due to interaction of multiple genetic and environmental factors.

4. Fetal Hydantoin Syndrome - A condition with the characteristic pattern of mental and physical birth defects from maternal use of Phenytoin (anti-convulsant or anti-seizure drug).

5. Meckel-Gruber Syndrome - It is a rare autosomal lethal malformation characterized by typical manifestations of occipital encephalocele (swelling of different sizes over the occipital bone), polycystic kidneys (a genetic condition characterized by the development of multiple cysts leading to enlargement and eventual loss of function of the kidney), and postaxial polydactyly (a complex congenital condition leading to the formation of an extra digit fibular or ulnar side of the foot and hand).

6. Jarcho-Levin Syndrome - It is a rare, congenital disorder that represents a spectrum of clinical and radiographic abnormalities of the spine and chest.

7. Multiple Lentigines Syndrome - It is a genetic condition characterized by abnormalities in the skin, face, head, inner ear, heart, and genitals. The condition is caused due to mutations in the PTPN11 gene; however, in a few individuals, mutations in the RAF1 gene have also been known to cause the abnormality.

8. Trisomy 13 and Trisomy 18 - Both of them are chromosomal abnormalities that cause intellectual disability and physical abnormalities.

9. Zellweger Syndrome - It is a genetic disorder that disturbs cellular function, leading to liver and kidney problems in newborns and difficulty in feeding and moving.

Along with the associated conditions, fetuses with SUA have also been known to have intrauterine growth retardation (IUGR) due to placental insufficiency. Other complications seen in SUA are preterm labor, fetal and neonatal death, placental anomalies, and abnormal umbilical cord insertions.

How Is Single Umbilical Artery Diagnosed?

The International Society of Ultrasound in Obstetrics and Gynecology recommends checking for the following during the first-trimester ultrasound-

1. The number of cord vessels.

2. Cord insertion to the umbilicus.

3. Cysts in the umbilical cord.

However, this procedure is not followed regularly because the sensitivity of the first-trimester sonogram to detect SUA is poor. Hence, the American Institute of Ultrasound in medicine recommends umbilical cord evaluation in the second and third trimesters.

• If an SUA is detected, it is important to look for other structural malformations in the fetus; a thorough fetal heart scan (i.e., four-chamber view, outflow tracts, and three-vessel view) is warranted.

• If the ultrasound cannot scan completely or associated malformations are noticed, an anatomy scan and fetal echocardiography should be done from a specialized center.

• If the second-trimester scan detects an isolated SUA, then there is no need for fetal echocardiography.

• Invasive testing with chromosomal evaluation is only recommended if the associated malformations are detected.

• While diagnosing isolated SUA, it is important to note the elevated maternal serum pregnancy-associated plasma protein A (PAPP-A) levels in the first trimester and alpha-fetoprotein (AFP) levels in the second trimester.

• A growth scan is recommended for fetuses with isolated SUA in 28 to 32 weeks to assess the risk of IUGR.

How Is Single Umbilical Artery Managed?

Delivery should be managed as per standard obstetric indications in normal-weight fetuses with isolated SUA and normal cord insertion. Neonates born with isolated SUA do not require special neonatal care, and the long-term outcomes can be expected to be the same as in neonates with a three-vessel cord.

A neonatologist should be consulted for neonates with SUA and other associated medical conditions.

Conclusion:

SUA is highly prevalent in twins and miscarriages; once diagnosed, the fetus should be screened for structural anomalies and associated chromosomal abnormalities. A SUA fetus must be kept under frequent surveillance even without abnormalities due to its high incidence of IUGR, preterm labor, and adverse fetal outcomes.