In recent decades prenatal screening tests have diffused widely in many western countries. Since the late 1980s it has been possible to screen for pregnancies at high risk for Down’s syndrome by using biomarkers (maternal serum alphafetoprotein and human chorionic gonadotropin) and taking the mother’s age into account (later in this text called serum screening) - low levels of AFP indicate increased risk for Down’s syndrome. The sensitivity of serum screening is 54 % to 59 % (depending on whether gestational age is estimated using dates or ultrasound), with a specificity of 95 % (see a comprehensive review in Wald et al., 1997). High levels of AFP indicate increased risk for neural tube defects.