Placental histology and vascular architecture in spontaneous twin anemia polycythemia sequence

OBJECTIVE: To investigate histological features of spontaneous twin anemia polycythemia sequence (TAPS) placentas. METHODS: This cohort study included consecutive spontaneous, untreated TAPS placentas collected at the Leiden University Medical Center between October 2020 and February 2025. Cases were excluded for missing pathology reports, intrauterine fetal demise, termination, intrauterine transfusion, or laser surgery. Gross and histological placental features and clinical characteristics were assessed per twin. Placentas were injected with color-coded dyes to identify vascular anastomoses and delineate donor and recipient territories via the vascular equator. RESULTS: Fifteen spontaneous, untreated TAPS placentas were analyzed. Median gestational age (GA) at diagnosis was 30.4 weeks (IQR 27.3-32.6) and at birth 34.7 weeks (IQR 33.9-36.0). Donor twins had lower birth weights than recipients (2000 vs 2310 g; p < 0.001), despite higher placental weights (385 vs 324 g; p < 0.001). All placentas showed arteriovenous anastomoses (median 3, IQR 2-7). Delayed villous maturation occurred in 87% of donor placentas, often with hydrops, whereas none were observed in recipients (p < 0.001). Accelerated maturation was present in 67% of recipient placentas but absent in donors (p < 0.001). Maternal vascular malperfusion occurred in 33% of donor placentas, frequently with ischemia, and in none of the recipients. Increased fetal vascularity was seen in 87% of recipient placentas only. CONCLUSION: Delayed villous maturation and hydrops in donor placentas likely reflect chronic fetal hypoxia due to anemia, which may contribute to growth restriction and adverse outcomes. Accelerated maturation in recipient placentas may represent an adaptive response, possibly explaining their more favorable outcomes.