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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.

Original publication




Journal article


Am J Obstet Gynecol

Publication Date





1761 - 1767


Adolescent, Adult, Chorionic Gonadotropin, beta Subunit, Human, Down Syndrome, False Positive Reactions, Female, Humans, London, Maternal Age, Middle Aged, Neck, Predictive Value of Tests, Pregnancy, Pregnancy Trimester, First, Pregnancy-Associated Plasma Protein-A, Prenatal Diagnosis, Prospective Studies, Ultrasonography