Down syndrome screening in the first and second trimesters: what do the data show?

Recent advances in Down syndrome screening are providing women with additional options in how they are assessed for this condition. First trimester screening tests are at least as accurate as traditional second trimester testing and provide results earlier in a woman's pregnancy. Combining first and second trimester results increases the sensitivity and specificity of screening and reduce the number of women requiring invasive testing. This review summarizes and critiques the available data on Down syndrome screening and compares different testing algorithms. Meta-analyses of first trimester screening techniques are presented. Nuchal lucency measurements and other first trimester ultrasound markers, first trimester serum markers, and methods for combining these screens are addressed.