Prospective first-trimester screening for trisomy 21 in 30,564 pregnancies.

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 ss- 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 false-positive 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.