Second-trimester genetic sonography after first-trimester combined screening for trisomy 21.

OBJECTIVE: The purpose of this study was to evaluate the trisomy 21 screening performance of the first-trimester combined test followed by second-trimester genetic sonography.

METHODS: This retrospective cohort study included all women with singleton pregnancies undergoing combined screening followed by genetic sonography at 17 to 21 weeks from January 1, 2005, to January 31, 2008. Combined test trisomy 21 risks were multiplied by positive or negative likelihood ratios based on the second-trimester sonographic findings to determine the final trisomy 21 risk. Sonography was evaluated as the second part of (1) a stepwise sequential test applied to combined screen-negative pregnancies and (2) an integrated test applied to all combined screen patients regardless of the latter results. A final trisomy 21 risk of 1:270 or higher was considered screen-positive.

RESULTS: A total of 2231 pregnancies underwent combined screening, which detected 7 of 8 Down syndrome cases (87.5%) at a 9.6% screen-positive rate. A total of 884 of these patients (39.6%), including 2 having fetuses with Down syndrome, had genetic sonography. Combined screening detected 1 of these trisomy 21 fetuses (50%) at a 15.7% screen-positive rate. Integrated ultrasound-based aneuploidy screening detected both trisomy 21 cases (100%) at a 22.7% screen-positive rate, whereas stepwise sequential ultrasound-based aneuploidy screening also detected both trisomy 21 fetuses (100%) but at a 28.3% screen-positive rate (P< .0001).

CONCLUSIONS: Second-trimester genetic sonography after first-trimester combined screening may improve trisomy 21 detection at the expense of increasing screen-positive rates.