Abstract
Down's syndrome has an incidence of about 1·3 per 1000 births. Antenatal testing for this disorder on the basis of maternal age has been common practice (increasing age is associated with increasing risk of having an affected pregnancy). Since the late 1980s it has been possible to calculate a pregnancy-specific risk using maternal age and serum marker levels, but the absence of a national screening policy in the UK has resulted in a diversity of screening practice. Screening may take place in the first or second trimester of pregnancy, using maternal age in combination with up to six serum markers, with or without ultrasound markers. Additionally, some units offer the screen only to women in specific age groups. Risk and marker levels are affected by a number of factors for which adjustments in the calculation may be made. Laboratories offering screening must also establish robust monitoring and quality management systems to ensure continued confidence in the reported result. Current diagnostic techniques require the use of invasive procedures which pose a risk to the fetus and the mother. Future screening may combine new technologies with the identification of fetal cells in maternal blood to reduce or abolish the need for these procedures.