Background:The umbilical cord (UC) is the lifeline between the fetus and placenta. It allows gas and nutrient exchange. Diabetes mellitus is the most common metabolic disorder that occurs during pregnancy, it has complications on both mother and fetus. The changes in the structure of UC with diabetes may have effect on the viability of the fetus. Aim of the Work: Is to study the effect of gestational diabetes on the histological structure of UC. Patients and Methods: The umbilical cord biopsies were collected immediately after delivery from 30 full term pregnant women, 15 normal pregnant females (Control group), and 15 pregnant women had gestational diabetes (Diabetic group). Informed written consent was taken from all pregnant women sharing in this study. These biopsies were processed for light and electron microscopic studies. Also morphometric and statistical analysis for the data were done. Results: Light microscopic study of diabetic UC samples showed degeneration of Wharton's jelly fibers with formation of honey comb like empty spaces. Erosion of endothelial cells lined the umbilical vessels; with separation of their smooth muscle fibers were observed. There were degeneration of collagen fibers and decrease amount of elastic fibers in the wall of umbilical vessels. Morphometric studies showed significant decrease in the intima and media thickness in diabetic UC arteries compared to UC control arteries. The UC vein of diabetic samples showed significant dilatation of the lumen and thinner wall compared to that of control group. Conclusion:The present study proved that gestational diabetes has many injurious effects on histological structure of UC.