Objective: This study was undertaken to evaluate the performance of a 1- stop clinic for first- trimester assessment of risk (OSCAR)for trisomy 21 by a combination of maternal age, fetal nuchal translucency (NT) thickness, and maternal serum- free β - human chorionic gonadotrophin (hCG) and pregnancy- associated plasma protein- A (PAPP- A). Study design: OSCAR was carried out in 30,564 pregnancies at 11 to 13+ 6 weeks. Patient- specific risks for trisomy 21 and detection and falsepositive rates were calculated. Results: The median maternal age was 34 (range 15- 49) years. Chromosomal abnormalities were identified in 330 pregnancies, including 196 cases of trisomy 21. The estimated risk for trisomy 21 was 1 in 300 or greater in 7.5% of the normal pregnancies, in 93.4% of those with trisomy 21 and in 88.8% of those with other chromosomal defects. Conclusion: The most effective method of screening for chromosomal defects is by first- trimester fetal NT and maternal serum biochemistry. Objective: This study was undertaken to evaluate the performance of a 1- stop clinic for first- trimester assessment of risk (OSCAR) for trisomy 21 by a combination of maternal age, fetal nuchal translucency (NT) thickness, and maternal serum-free beta Study design: OSCAR was carried out in 30,564 pregnancies at 11 to 13+ 6 weeks. Patient-specific risks for trisomy 21 and detection and false positive. Results: The median maternal age was 34 (range 15-49) years. Chromosomal abnormalities were identified in 330 pregnancies, including 196 cases of trisomy 21. The estimated risk for trisomy 21 was 1 in 300 or greater in 7.5% of the normal pregnancies, in 93.4% of those with trisomy 21 and in 88.8% of those with other chromosomal defects. Conclusion: The most effective method of screening for chromosomal defects is by first- trimester fetal NT and maternal serum biochemistry.