Subinvolution of the uteroplacental arteries in the human placental bed.

Subinvolution of the uteroplacental arteries of the placental bed is a recognized cause of post partum haemorrhage causing significant morbidity. Whilst the physiological changes in these arteries during pregnancy and the part played by endovascular trophoblast migration are well documented, the sequence of events during involution and the pathophysiology of subinvolution are unknown. Using immunohistochemical techniques we have studied uteroplacental arteries in the placental bed in 25 cases of post partum haemorrhage and compared the subinvoluted vessels with normally involuted vessels. Non-involuted vessels were present in 22 test cases; these abnormal vessels were filled with thrombus and no endothelial lining was detected. Extravillous perivascular trophoblast was usually present in the walls of these abnormal vessels and in some cases was seen in an endovascular position. Subinvolution of placental site vessels may represent an abnormal interaction between maternal uterine cells and fetal trophoblast.