Screening for Down's Syndrome has been shown to be effective at 10 weeks of pregnancy. A multicentre study (the first trimester serum screening study) has shown that there are two biochemical markers of choice at this time in pregnancy, namely pregnancy associated placental protein A (PAPP-A) and the free beta-sub-unit of human chorionic gonadotrophin (free beta-hCG). When used together with maternal age these two biochemical markers have an estimated detection rate of 62% and a 5% false-positive rate. The results are consistent with those obtained from a systematic review of the world literature. Other markers are less predictive of Down's syndrome though there is still some uncertainty over the value of dimeric inhibin-A at 10 weeks of pregnancy. Nuchal translucency measurement, from an ultrasound examination performed at about 10 weeks of pregnancy, is associated with Down's syndrome and is emerging as an important potential screening marker. At present there is uncertainty over its quantitative performance and performance when combined with biochemical markers. The resolution of these issues is currently the subject of active research. Ten week screening for Down's syndrome is an advance that is now technically possible though there is still insufficient information to justify its use in routine screening practice.