Chorionic villus sampling (CVS) or amniocentesis for fetal sex determination is 
generally the first step in the prenatal diagnosis of X-linked genetic disorders 
such as Duchenne muscular dystrophy (DMD). However, non-invasive prenatal 
diagnostic (NIPD) techniques such as measurement of cell-free fetal DNA (cffDNA) 
in maternal plasma are preferable given the procedure-related miscarriage rate 
of CVS. We determined fetal sex during the first trimester using a quantitative 
real-time polymerase chain reaction (PCR) assay of cffDNA in pregnant carriers 
of DMD. The fetal sex was confirmed by amniocentesis karyotype analysis and 
multiplex ligation-dependent probe amplification (MLPA) at 16 weeks. This 
procedure may avoid unnecessary CVS or amniocentesis of female fetuses.