Maternal dried whole-blood specimens were collected prospectively from 2010 singleton pregnancies between 9 + 0 and 13 + 4 weeks that included 18 chromosomally abnormal pregnancies (11 Down's syndrome, four trisomy 18, two trisomy 13 and one triploidy). A subset of 744 pregnancies underwent ultrasound nuchal translucency measurement and included seven Down's syndrome, four trisomy 18, two trisomy 13 and one triploidy. Patients were evaluated for risk of Down's syndrome and trisomy 18 based on biochemistry (free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A), nuchal translucency and the combination of both. In prospective biochemical screening, false-positive rates for Down's syndrome and trisomy 18 were 5.1% (66/1297) and 1.9% (25/1297) in women < 35 years of age and 14.2% (99/695) and 1.6% (11/695) in women > or = 35 years of age, respectively. The detection efficiency of aneuploidy was 6/6 (100%) in women < 35 years and 11/12 (92%) in women > or = 35 years. Nuchal translucency measurement alone detected 57% (8/14) of cases of aneuploidy at a 5.8% (42/730) false-positive rate. Modelling with the age distribution of live births, a 5% false-positive rate resulted in Down's syndrome detection efficiency of 61% by biochemistry, 73% by nuchal translucency and 87% by combining both methods. The data in this study demonstrate that combined biochemical and ultrasound evaluation for Down's syndrome and other chromosomal abnormalities in the first trimester of pregnancy yield a detection capability that may exceed that of current second-trimester prenatal screening protocols. The potential for enhanced detection coupled to an earlier alert of fetal complications could represent a substantial advantage to both clinician and patient.