Mother-fetus exchanges at the placental level are found to be altered in women affected by hypertensive or diabetic pregnancies following the onset of microenvironmental, circulatory, trophic or tissue disorders. Our aim was therefore to assess the alterations occurring within the umbilical cord, particularly its venous endothelial component and underlying smooth muscle layer, using transmission (TEM) and scanning electron microscopy (SEM) and immunohistochemical analyses. Immunohistochemical data appear to support the ultrastructural evidence for an activated state of these vascular structures, in both conditions (hypertension and diabetes). Furthermore, mainly during diabetic pregnancies, extracellular matrix molecules such as tenascin and fibronectin also quantitatively increase at the vein wall level. The umbilical cord seems to be a structure capable of responding actively to abnormal microenvironmental conditions which seriously threaten the health of the fetus and also the mother herself.