Normalized chromosomal read count in relation to the fetal fraction. Normalized chromosomal read count in relation to the fetal fraction (SeqFF) for chromosomes 13 (A), 18 (B), 21 (C), and 7 (D). The X-axis depicts the fetal fraction and the Y-axis the normalized chromosomal read count. Horizontal and diagonal lines, respectively, mark the predicted normal and trisomic normalized read counts (full lines) and ±3 SD as measured in the normal cases (X-axis and dashed lines). Values within 3 SD from expected trisomy counts are plotted as blue triangles. Values outside the area delineated by the X-axis and dashed lines represent cases with |Z| > 3 and |TriZ| > 3. These are plotted as red circles and indicate pregnancies at risk for fetoplacental mosaicism. The arrows indicate a set of discordant twins. Reproduced from Brison et al (2019) with permission from Wiley-Blackwell Publishing44