Next-Generation–Sequencing-Based Noninvasive Prenatal Diagnosis - Next Generation Sequencing Technologies in Medical Genetics - page 44

Fig. 5.1 Schematic
illustration for the
noninvasive prenatal
detection of fetal
chromosomal aneuploidy
using next-generation-
sequencing (adapted from
Chiu et al. 2008 )
mapped to each chromosome, %chrN, is determined by dividing the U0-1-0-0
count of a specifi c chromosome by the total number of U0-1-0-0 sequence reads
generated in the sequencing run for the tested sample. Then, the z -score, defi ned as
the number of standard deviations from the mean of a reference data set, of %chr21
of the tested sample is calculated to determine whether or not it was a T21 pregnancy.
Hence, for a T21 fetus, a high z -score for %chr21 is expected when compared with
the mean and standard deviation of %chr21 values obtained from maternal plasma
of euploid pregnancies.
Despite the potential applications of the method, it is important to be aware of the
assumptions that are made for effective noninvasive prenatal fetal chromosomal