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 quantify immunovascular dysregulation. Methods: This retrospective study included all OD triplet pregnancies with placental examination performed during 24 years at a tertiary referral center. Maternal, obstetric, fetal, neonatal, and pathological variables were analyzed at the pregnancy level. Histological and immunohistochemical features previously shown to differ between OD and non-OD pregnancies were grouped into six domains: innate immunity, adaptive immunity, checkpoint regulation, vascular remodeling, complement activation, and trophoblastic behavior. Binary thresholds, immunoreactive scores or established morphological cut-offs, were applied to construct a 20-point score classified into three grades. Results: Forty-five OD triplet pregnancies were analyzed. Intra-pregnancy concordance for PARS components was high, with intraclass correlation coefficient ≥0.70 in 87.3% pregnancies. Increasing PARS grades demonstrated a clear clinical gradient. Grade 3 pregnancies had significantly lower birthweight, higher rates of prematurity (<34 weeks), and increased fetal growth restriction. Placental weight decreased progressively with higher PARS. Histologically, grade 3 placentas showed significantly increased accelerated villous maturation and intervillous fibrin deposition. Conclusions: PARS integrates immune and vascular placental lesions into a structured and reproducible framework that correlates with clinically relevant perinatal outcomes and may support future clinical risk stratification, although further validation in larger, multicenter prospective cohorts is required.
Pena‐Burgos et al. (Thu,) studied this question.