Thanks for your query.
Single umbilical artery
means there is only one artery one vein otherwise there are two artery one vein normally. It is rarely problem associated with congenital abnormalities (cardiac, neural and renal) and chromosomal defect in the fetus.
Half and two third baby born normal with this problem other have congenital and chromosomal deformity
.You should consult a gynecologist and your foetus detailed ultrasonogrphy done to rule out any congenital deformity.
God bless you.