Fetal sex determination

The detection of genetic material from the Y chromosome in pregnant women’s blood made finding new ways of non-invasive investigation in male foetuses possible, right from the first weeks of pregnancy. Using a quantitative real-time PCR system, it was estimated that free fetal DNA represents around 3.4% of total DNA in maternal blood at the end of the 1st trimester of pregnancy. At the end of the pregnancy, this value increases to 6.2% (Lo et al., 1998).

A number of studies have shown the possibility of identifying free DNA in maternal plasma from the 5th week of pregnancy. Hence, it is possible to determine fetal gender (before any possible morphological differentiation by ultrasound) and antenatally investigate risk pregnancies for X-linked diseases, such as haemophilia or Duchenne muscular dystrophy.

Another application which arises from knowing fetal gender refers to situations where pregnant women carry congenital adrenal hyperplasia (recessive autosomal disease which affects 1 in 5000 persons) and in which identifying male foetuses will make it unnecessary to treat the pregnant woman with corticoids (Chiu et al., 2002a). The usual treatment for congenital adrenal hyperplasia in risk families consists in administering dexamethasone to pregnant women, preferably from the 8th-9th week of pregnancy, at a time when knowing fetal gender is still not possible. Despite the treatment being effective in female affected foetuses, in many cases early exposure to dexamethasone is unnecessary. Therefore, early determination of fetal gender by fetal DNA testing in maternal plasma may represent a first approach to detection and follow-up of such cases.