**4.1. Hypoplastic umbilical artery**

A hypoplastic umbilical artery has a smaller diameter than the contralateral artery, showing by ultrasonography an artery-to-artery diameter difference of more than 50% [37] (**Figure 7**).

It seems that the hypoplastic umbilical artery represents a mild form of the single umbilical artery. Described anomalies include trisomy 21, polyhydramnios, congenital heart disease, stillbirth, trisomies, and fetal growth restriction. The presence of discordant umbilical arteries is a sign of different umbilical artery blood flow indices and of placental disease [38]. This condition increases the risk of IUGR, placental infarction, umbilical cord hematoma, and abnormal umbilical cord insertion. It is also known that the fetal prognosis is better for hypoplastic umbilical arteries compared with SUA syndrome [37].

Karyotyping is not indicated in isolated hypoplastic umbilical artery because there is no evidence of increased risk of chromosomal defects.

**Figure 7.** Hypoplastic umbilical artery.
