This is a prospective observational study enrolling all comers who attend the prenatal clinic for first trimester assessment of the risk for Down syndrome. All women are providing medical and obstetric history, demography and blood samples along with Doppler pulsatility Index during the first and the second trimester at GA 10-13 weeks and 21-23 weeks. When possible - blood will be drawn at hospital admission for delivery. Measurements of sonography and serum markers are done blinded to pregnancy outcome.