These observations led to a number of screening studies in the second trimester, which assessed if it is possible to predict those pregnancies destined to have complications of impaired placentation. These studies showed that women with increased impedance to uterine artery blood flow have an increased risk of developing pre-eclampsia, and detection can be further increased by using angiogenic factors (That are involved in the pathogenesis of Pre- eclampsia and could be useful for early prediction of the disease: VEGF, PIGF, PP13, sFLT1) in combination with uterine arteries Doppler study in the second trimester.