**2. Abnormal number of vessels**

Sometimes, during pregnancy, changes in the number of umbilical vessels may occur. Abnormal number of umbilical cord vessels includes: two-vessel cord (single umbilical artery), four-vessel cord (two veins and two arteries, one vein and three arteries), five and more vessels cord (numerous variations in conjoined twins), umbilical cord that does not keep the same number of vessels at the fetal and placental extremity [3].
