Placental villi with an increased stromal complement of lymphocytes and other mononuclear cells, often with destruction of local villous vessels and syncytial injury with perivillous fibrin deposition. Chronic villitis can be focal or diffuse.

T-cell immune response with diffuse perivillous fibrin deposition associated with intrauterine growth restriction, pre-term birth and recurrent fetal loss.

Villitis of unknown etiology (synonymous with villitis of unknown etiology) is a histologic diagnosis and, although it may have a variable distribution, evidence indicates that 3 parenchymal blocks will identify 62% of villitis, reaching an asymptote of 6 and 7 blocks identifying 85% of villitis. Villitis of unknown etiology by definition excludes those cases where an etiology is identified, such as viral or acute infections, and thus is preferred to the term nonspecific chronic villitis. It is usually lymphohistiocytic: although the presence of rare plasma cells does not exclude the diagnosis, thorough evaluation for an infectious etiology, such as cytomegalovirus, is required in a predominantly plasma cell villitis.