Supplementary Material for: Collaboration on treatment of Twin-to-twin transfusion (TTTS) syndrome in a borderless Nordic Center

Introduction
The Nordic Fetal Therapy Alliance centralizes treatment of TTTS to ensure high-quality fetal therapy. This study reports the outcomes of treated TTTS cases and highlights the importance of international collaboration.

Methods
We conducted a prospective observational study on all pregnancies undergoing TTTS surgery between May 2019 and April 2022 in the Nordic countries. The primary outcome was perinatal survival at 28 days. Secondary outcomes were gestational age (GA) at delivery, procedure-related complications, and mode of delivery.

Results
A total of 200 cases underwent fetal surgery; 185 were treated with fetoscopic laser photocoagulation (FLP), and 15 with primary selective reduction by cord occlusion (CO). FLP resulted in at least one surviving neonate in 86% and in at least two survivors in 64%. Median GA at surgery was 20+1 weeks (range 15+1; 29+0), and median GA at delivery was 32+2 weeks (range 24+3; 41+0). In the CO group, 80% had at least one surviving neonate. Preterm premature rupture of membranes occurred in 13.6% within the FLP group and none in the CO group. Post-laser TAPS occurred in 5.4%.

Conclusion
Collaboration across borders on TTTS treatment in the Nordic region is possible, and outcome results are comparable to other International Centers.