Insights in fetal growth trajectories of spontaneous twin anemia polycythemia sequence twins

Introduction: Twin anemia polycythemia sequence (TAPS) is a monochorionic twin condition, where imbalanced blood flow causes anemia in the donor twin and polycythemia in the recipient twin. While TAPS donors are frequently small for gestational age (SGA, birth weight <10th centile), fetal growth patterns are yet poorly understood. Methods: We conducted a retrospective study of all spontaneous TAPS pregnancies diagnosed at Leiden University Medical Center between 2002 and 2024. Fetal growth was assessed by estimated fetal weight (EFW), with fetal growth restriction (FGR) defined as an EFW <10th centile. Secondary outcomes included SGA at birth and placental share distribution. Results: A total of 45 consecutive spontaneous TAPS pregnancies were included. Donors consistently had lower EFW than recipients. At 20 weeks, FGR was present in 42% (5/12) of donors versus 0% (0/11) of recipients (p<0.001). No recipients developed FGR before 24 weeks. By 30 weeks, FGR affected 55% (6/11) of donors and 9% (1/11) of recipients (p=0.028). At birth, 58% (22/38) of donors had a larger placental share than their cotwin. Conclusion: Despite a slightly larger placental share, TAPS donor twins are at higher risk of FGR. At birth, three out four TAPS donors are SGA compared to one out of five recipients.