The pregnancy complicated by uncontrolled maternal diabetes affected the maternal health and the placenta experiences a diversity of structural and functional changes. The study aims to explore the possible adverse effects of Gestational Diabetes Mellitus (GDM) on the morphometric of umbilical cord and its vessels. The study carried on fifty placentas with its cords of normal pregnancy were without any known pre-existing disease and 50 placentas with its cords from mothers complicated by GDM (untreated). Gestational diabetes mellitus group was selected from Gynecology and Obstetrics Department of AL-Hilla Teaching Hospital, Iraq. The gross examination showed that the number of cords with Single Umbilical Artery (SUA) was 13 out of 50 cords in pregnant women with gestational diabetes compared with normal pregnancy. All the histomorphmetric parameters of cord with SUA as the circumference, diameters, cross-section area of cord, CSA of whrtons jelly and histomorphometric umbilical vessels of SUA and vein of GDM group were decreased when compared to the normal group. The GDM has adverse perinatal outcome as respiratory distress, cardiovascular anomalies and still birth. It can be concluded that the lean cord with single umbilical artery were related with GDM and associated with adverse fetal outcome.