[Detection of cell-free fetal DNA in the urine of pregnant women by nested polymerase chain reaction].

OBJECTIVE: To investigate the clinical feasibility of fetal sex determination by using nested polymerase chain reaction (PCR) to detect cell-free fetal DNA in the urine of pregnant women. METHODS: The urine specimens of 40 healthy predelivery women were centrifuged. The DNA in the specimens of supernatant was extracted by using of QIAamp Viral RNA Mini Kits. Nested PCR was used to specifically amplify the SRY gene, 300 bp in length, on Y chromosome in order to detect the fetal DNA from the women bearing male fetuses. One specimen of normal man's urine and one specimen of non-pregnant woman's urine were used as controls for each specimen from pregnant women. The results were confirmed by the sexes of neonates examined after birth. RESULTS: Amplified band of SRY was found in 6 specimens. Sex examination after delivery showed 21 male neonates and 19 female neonates. The positive rate of detection of SRY gene was 28.6% (6/21) and the false negative rate was 71.4%, and false positive rate was 0. Five out of the 6 cases with positive results showed positive 2 times and one out of the 6 cases only showed positive at the second time. CONCLUSION: There is cell-free fetal DNA in the urine of pregnant women. Nested PCR is useful in detection of fetal sex, however, still with a rather low positive rate.