First- and second-trimester screening: detection of aneuploidies other than Down syndrome.

OBJECTIVE: To evaluate the performance of first- and second-trimester screening methods for the detection of aneuploidies other than Down syndrome.

METHODS: Patients with singleton pregnancies at 10 weeks 3 days through 13 weeks 6 days of gestation were recruited at 15 U.S. centers. All patients had a first-trimester nuchal translucency scan, and those without cystic hygroma had a combined test (nuchal translucency, pregnancy-associated plasma protein A, and free beta-hCG) and returned at 15-18 weeks for a second-trimester quadruple screen (serum alpha-fetoprotein, total hCG, unconjugated estriol, and inhibin-A). Risk cutoff levels of 1:300 for Down syndrome and 1:100 for trisomy 18 were selected.

RESULTS: Thirty-six thousand one hundred seventy-one patients completed first-trimester screening, and 35,236 completed second-trimester screening. There were 77 cases of non-Down syndrome aneuploidies identified in this population; 41 were positive for a cystic hygroma in the first trimester, and a further 36 had a combined test, of whom 29 proceeded to quadruple screening. First-trimester screening, by cystic hygroma determination or combined screening had a 78% detection rate for all non-Down syndrome aneuploidies, with an overall false-positive rate of 6.0%. Sixty-nine percent of non-Down syndrome aneuploidies were identified as screen-positive by the second-trimester quadruple screen, at a false-positive rate of 8.9%. In the combined test, the use of trisomy 18 risks did not detect any additional non-Down syndrome aneuploidies compared with the Down syndrome risk alone. In second-trimester quadruple screening, a trisomy 18-specific algorithm detected an additional 41% non-Down syndrome aneuploidies not detected using the Down syndrome algorithm.

CONCLUSION: First-trimester Down syndrome screening protocols can detect the majority of cases of non-Down aneuploidies. Addition of a trisomy 18-specific risk algorithm in the second trimester achieves high detection rates for aneuploidies other than Down syndrome.

LEVEL OF EVIDENCE: II.