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In fetuses at risk for single-gene (Mendelian) disorders, prenatal genetic diagnosis usually requires amniocentesis or chorionic villus sampling to obtain fetal cells for DNA analysis. Noninvasive procedures, such as serum screening and ultrasound, are not usually helpful. These researchers evaluated the effectiveness of contemporary ultrasound and molecular analysis of cell-free fetal DNA in maternal plasma to determine the sex of fetuses at risk for hemophilia, an X-linked recessive disorder that ordinarily affects only males. (If the fetus is female, a woman can avoid invasive diagnostic testing.) Both tests were performed between 11 and 14 weeks’ gestation on 10 women who were obligate heterozygotes for hemophilia and whose male offspri…