A study measured the concentrations of angiogenic and anti-angiogenic factors and showed that twin pregnant women with TTTS had higher levels of Endoglin and VEGFR-1 compared to those without TTTS while the placental growth factor (PlGF) levels were lower in the group with TTTS compared to those without TTTS 24. This evidence concerns the gene FLT1 and twin to twin transfusion syndrome.