Objective: The clinical application of first-trimester aneuploidy screening remains controversial in the United States. The aim of our study was to evaluate the performance of maternal age, fetal nuchal translucency measurements, pregnancy-associated plasma protein A, and free beta-human chorionic gonadotrophin used in aneuploidy screening in a single institution outside of a clinical trial. Study design: Four thousand eight hundred eighty three patients underwent first-trimester aneuploidy screening at 11 to 13 6/7 weeks of gestation (fetal crown-rump length 45 mm to 84 mm) at our institution between January 2003 and September 2004. Measurement of nuchal translucency was performed according to the Fetal Medicine Foundation standards and was included in the overall risk assessment performed by NTD Laboratories. Measurement of pregnancy-associated plasma protein A and free beta-human chorionic gonadotrophin on maternal dried whole blood samples was conducted by NTD Laboratories and was reported as gestational-specific multiples of the median adjusted for ethnicity. Risk adjustment for trisomy 21 and trisomy 18 was done with a standard algorithm using maternal age, serum biochemistry, and nuchal translucency. Only singleton gestations (N = 4615) were included in the analysis. Results: The median maternal age was 33.0 years (interquartile range 31.0 to 36.0) and the median crown-rump length was 61.2 mm (interquartile range 55.7 to 67.2) at the time of screening. There were a total of 22 fetuses diagnosed with trisomy 21 and 8 with trisomy 18. The detection rates for trisomy 21 for a 5%false-positive rate and 1%false-positive rate were 90.9%(20 of 22) and 77.3%(17 of 22), respectively. Similarly, the detection rates for trisomy 18 at a 5%false-positive rate and a 1%false-positive rate were 100%(8 of 8) and 100%(8 of 8), respectively. Conclusion: Non-investigational use of first-trimester aneuploidy screening for trisomy 21 and trisomy 18 can replicate results from investigational trials. Objective: The clinical application of first-trimester aneuploidy screening remains controversial in the United States. The aim of our study was to evaluate the performance of maternal age, fetal nuchal translucency measurements, pregnancy-associated plasma protein A, and free beta-human chorionic gonadotrophin used in aneuploidy screening in a single institution outside of a clinical trial. Study design: Four thousand eight hundred eighty three patients underwent first-trimester aneuploidy screening at 11 to 13 6/7 weeks of gestation (fetal crown-rump length 45 mm to 84 mm) at our institution between January 2003 and September 2004. Measurement of nuchal translucency was performed according to the Fetal Medicine Foundation standards and was included in the overall risk assessment performed by NTD Laboratories. Measurement of pregnancy-associated plasma protein A and free beta -human chorionic gonadotrophin on maternal dried whole blood samples was conducted by NTD Laboratories and was reported as gestational-specific multiples of the median adjusted for ethnicity. Risk adjustment for trisomy 21 and trisomy 18 was done with a standard algorithm using maternal age, serum biochemistry, and nuchal translucency. Only singleton gestations (N = 4615) were included in the analysis. Results: The median maternal age was 33.0 years (interquartile range 31.0 to 36.0) and the median crown-rump length was 61.2 mm (interquartile range 55.7 to 67.2) at the time of screening. There were a total of 22 fetuses diagnosed with trisomy 21 and 8 with trisomy 18. The detection rates for trisomy 21 for a 5% false-positive rate and 1% false-positive rate were 90.9% (20 of 22) and 77.3% (17 of 22), respectively. Similarly, the detection rates for trisomy 18 at a 5% false-positive rate and a 1% false-positive rate were 100% (8 of 8) and 100% (8 of 8), respectively. Conclusion: Non-investigational use of first- trimester aneuploidy screening for trisomy 21 and trisomy 18 can replicate results from investigational trials.