Efficacy of the genetic sonogram in a stepwise sequential protocol for down syndrome screening.

OBJECTIVES: The purpose of this study was to evaluate the efficacy of the genetic sonogram in Down syndrome screening for women who have received the stepwise sequential test.

METHODS: This retrospective cohort study included women with singleton pregnancies who underwent stepwise sequential (first-trimester combined and second-trimester serum) screening and then had a genetic sonogram between March 2005 and January 2010. Stepwise sequential Down syndrome risks were multiplied by either a positive or negative likelihood ratio based on the second-trimester sonographic findings to determine the final Down syndrome risk. A final Down syndrome risk of 1:270 or higher was considered screen positive.

RESULTS: A total of 6286 women fulfilled our criteria, including 17 with Down syndrome-affected fetuses. After stepwise sequential testing, the Down syndrome detection rate was 88.2% (15 of 17), and after the genetic sonogram, there was a non-significant reduction in detection to 82.4% (14 of 17; P > .05). For the 6269 unaffected pregnancies, the genetic sonogram converted 58 screen-negative results (1%) to positive and 183 screen-positive results (3.1%) to negative. The net effect was a change in the false-positive rate from 6.2% (390 of 6269) after stepwise sequential screening to 4.2% (266 of 6269) after the genetic sonogram.

CONCLUSIONS: The genetic sonogram should be applied cautiously for women who have received prior prenatal screening tests. Women with screen-positive results need to be counseled that a negative sonographic result can be falsely reassuring. Conversely, for women with screen-negative results who have a risk close to the cutoff, a sonographic examination could assist in the decision of whether to accept or reject amniocentesis.