The Role of Platelets and Fibrin in Uteroplacental Arteries in Normal Human Pregancy

In normal human pregnancy the uteroplacental vessels undergo extensive morphological adaptations to provide an increasing blood flow to the placenta. These involve platelets and fibrin adjacent to trophoblast invasion of spiral arteries.

In early pregnancy, single layers of platelets or small thrombi are seen between trophoblast cells which form a pseudo-endothelium of the spiral arteries. In the media trophoblast cells and fibrin are present in an amorphous matrix which replaces the musculo-elastic component of the vessel. In late pregnancy these vascular changes extend into the myometrium, fibrin deposition is increased and mature mural thrombi are found.

The fibrin deposition in uteroplacental arteries, is a physiological phenoncma of pregnancy. Fibrin may provide the dynamic structural support required in these vessels expanding to accommodate an increasing blood flow. Fibrin also enables the immediate collapse of the vessel following placental separation. Mural thrombosis in the arterial supplyprobably accounts for areas of ischemia and small placental infarcts found in normal pregnancy. Because of the large placental reserve, this minor degree of spiral artery occlusion does not appear to effect adversely the fetus.