Soluble VEGF-R1 levels were slightly increased in twin pregnant women with TTTS compared to those without TTTS [median (min-max): 702.6 (359.5-1322.1) vs. 565.5 (128.9-1381.7)] (P=0.057) (Figure 2). This evidence concerns the gene FLT1 and twin to twin transfusion syndrome.