Stepwise sequential screening for Down's syndrome (combined test associated with modified genetic sonography) in pregnant women with low risk for chromosomal disorders.

Objective: To assess the sensitivity (Sen) and false positive ratio (FPR) of stepwise sequential screening [1st step: combined test (CT), 2nd step: modified genetic sonography (major malformation and nuchal fold, MGS)] as a screening method for Down's syndrome (DS) in the general population of pregnant women.

Methods: Prospective study. During a 5-year study period (July 2005 to June 2010), 17,911 pregnant women were screened for DS using a stepwise sequential screening method (CT+MGS). We evaluated the Sen and FPR (95% CI) of the two chromosomal disorder screening methods for DS: CT and CT+MGS.

Results: Seventeen thousand nine hundred and eleven cases were analysed, including 67 with chromosome abnormalities and 45 with DS. The Sen of CT for DS was 80% (95% CI; 68.3-91.7) (36/45) with a FPR of 4.2% (95% CI; 3.9-4.5) (752/17, 866). The Sen of CT+MSG for DS was 93.3 (95% CI; 85.9-99) (42/45) with a FPR of 4.8% (95% CI; 4.5-5.1) (860/17, 866).

Conclusions: MGS coupled with CT increases the Sen of DS diagnosis by 13.3% (95% CI; 2.7-25.9), with an increase in FPR of 0.6% (95% CI; 0.5-0.7).