What is the definition or description of: single umbilical artery?

Umbelical cord: normally has 2 arteries and 1 vein 4 different kinds Type 1-Most common comprising 98% of cases has2 patent vessles 1 an artery of the allentoin and the vein from the left umbelical vein this form is most associated with genito urinary abnormality usually associated with pregnancies in extreme maternal age,smoking.diabetes ,hypertension