Maternal blood testing can accurately identify fetal sex

Clinical question Is maternal blood testing late in the first trimester accurate to determine fetal sex?

Bottom line Test results of maternal plasma for fetal DNA late in the first trimester to determine fetal sex are accurate. The main current indication of this testing is to avoid invasive testing of the female fetus for X-linked congenital disorders. (Level of evidence = 1b)

Synopsis The presence of cell-free fetal DNA originating from the placenta in the plasma of pregnant women was discovered in the 1990s. Early knowledge of fetal sex can be helpful; for example, to avoid invasive testing of female fetuses for X-linked conditions. A polymerase chain reaction (PCR) test of maternal plasma can determine the presence or absence of sequences specific to the Y chromosome or paternal alleles present in the father, but absent in the mother. These Dutch authors report the results of 201 cases tested from 2003 to 2009. Fetal sex was ascertained by karyotyping, ultrasonography, or after birth in 98% of cases. Ultrasonography prior to PCR testing was performed to confirm viable singleton gestation. The researchers used a minimal gestational age of 7 weeks for indication of risk for congenital adrenal hyperplasia and 9 weeks for all others to reduce the likelihood of false-negative results due to low levels of fetal DNA. Testing for presence of paternal sequences absent in the mother was also performed to confirm the presence of fetal DNA. There were no cases of misclassification of boys and girls. In all cases of inconclusive results (n = 10), the sex of the newborn was female.

Scheffer PG, van der Schoot CE, Page-Christiaens GC, et al. Reliability of fetal sex determination using maternal plasma. Obstet Gynecol. 2010;115(1):117-126.