Umbilical artery aneurysm is a rare condition associated with increased risk for aneuploidy and fetal demise.We report a case of umbilical artery aneurysm discovered at 27 weeks of gestation in one fetus of a dichorionic, diamniotic twin pregnancy. The patient was hospitalized to monitor for expansion of the aneurysm. Corticosteroids were administered, and, after genetic amniocentesis revealed a normal karyotype, cesarean delivery was performed at 28 2/7 weeks of gestation. Pathologic examination confirmed an umbilical artery aneurysm in the cord of the affected fetus.Given the high incidence of aneuploidy associated with umbilical artery aneurysm, it is important to consider karyotype analysis of the affected fetus. If a normal karyotype is identified, early delivery may be warranted to decrease the risk of fetal demise.