Fig. 1. Correlation between cell-free DNA (cfDNA) fraction and gestational age in 46 pregnancies with male fetuses (R²=0.1636, P<0.01). Unlike for male fetuses, for which Y chromosomes are effective markers, it is particularly difficult to quantitate the fetal fraction in female fetuses; there are no universal and reliable fetal markers available to estimate the fetal fraction in maternal plasma [15]. Therefore, cfDNA level was analyzed using only male fetuses.