Thrombosis of the umbilical artery was diagnosed in utero in two cases at the echography examination performed during the third trimester of gestation. The infants were born live and in good health. This is the first report of such antenatal diagnosis. The lack of prospective studies hinder the evaluation of fetal morbidity and mortality. Nevertheless, umbilical artery thrombosis is a high risk obstetrical situation since the potential risk of another thrombus in the second umbilical artery would lead to fetal death in utero. Careful monitoring is required with extraction of the fetus as soon as term allows or whenever the elements compromise fetal development.