Second trimester soft markers: still worth to be mentioned?

Despite the widespread use of cell-free DNA in screening for trisomy 21, soft markers continue to be assessed in the second trimester, leading to confusion about how they affect the risk of trisomy 21, especially after an earlier screening test such as cell-free DNA screening. In this review, we provide an overview of commonly used second trimester soft markers, explain how they can be used to calculate the risk for trisomy 21, and discuss what other chromosomal or structural abnormalities might be associated with these markers. We especially focus on pathogenic copy number variants as these, in aggregate, are common and are very difficult to detect using cfDNA or even standard karyotyping.