FLT1 and twin to twin transfusion syndrome: In the first trimester, we observed that soluble VEGF-R1 levels were significantly higher in monochorionic twin pregnant women with and without TTTS [median (min-max): 763.3 (376.9-1940.6) and 779.2 (157.1-1746.5), respectively] compared to those in single pregnant women [median (min-max): 560.5 (326.9-1461.6)] (P=0.004 and 0.001, respectively).