Is the Placenta an Immune Battlefield in Oocyte Donation? Histological Evidence of Graft-Versus-Host-like Phenomena in Triplet Pregnancies and the Development of PARS (Placental Allograft Rejection-like Score)

Background/Objectives: Oocyte donation (OD) pregnancies involve complete maternal–fetal genetic disparity and are associated with increased placental dysfunction and adverse perinatal outcomes. However, a unified histopathological framework to characterize alloimmune-mediated placental injury in OD gestations is lacking. This study evaluates immune and vascular alterations in OD triplet placentas and proposes a structured scoring system, the Placental Allograft Rejection-like Score (PARS), to qu