Involution of Implantation Site and Retained Placenta

Abstract

Pathologists rarely obtain a postpartum uterus to enable a detailed study of the involutional changes that take place at the former site of implantation. Therefore, involution of the normal placental site has been studied by only a few investigators. Normally, the postpartum lochia contain the decidual remnants, including perhaps some of the degenerating remnants of the vasculature that had previously undergone the so-called physiological changes of pregnancy. Only when significant postpartum hemorrhage occurs and hysterectomy then becomes necessary is the pathologist asked to seek the cause of the bleeding. He or she may then find remains of villi, incompletely thrombosed vessels, “placental polyps,” and some degree of inflammatory reaction. Frisoli (1981) suggested that retained placental tissue is found in about 50% of such cases at curettage. The curettings and postpartum uteri are difficult tissues to study objectively because most pathologists have little experience with the normal, complex process of placental site involution. Williams (1931), in a classical paper, attempted to rectify this situation. His study should be read before any interpretation of such a postpartum uterus is undertaken. It must also be recognized that in 85% of normal, delivered placentas the decidua basalis shows foci of polymorphonuclear leukocyte infiltration (Schneider, 1970). These cells are part of an apparently normal process of implantation and are not considered an expression of deciduitis or infection. Nevertheless, postpartum hemorrhage and subinvolution are nearly always associated with significant inflammation, if only because the cervix remains open and patulous.