A non-invasive test can tell whether a 7 week fetus is a boy or girl, researchers reported in JAMA (Journal of the American Medical Association). The authors say the test may help in the early diagnosis of genetic diseases on the X chromosomes, diseases that affect males only. Doctors today usually diagnose X-linked diseases before birth via amniocentesis or chorionic villus sampling, invasive tests that raise the risk of miscarriage.

The fetal gender test does not diagnose X-linked diseases. However, as it can determine the sex of the fetus early on, at 7 weeks, it could lead to a 50% reduction in the number of invasive tests for sex-specific diseases.

Dennis Lo led a research group in 1997 that demonstrated that fetal DNA is detectable in maternal blood very early during pregnancy via the presence of Y chromosome markers. Their findings have since been replicated several times by different research teams, at different stages of pregnancy. They used a variety of DNA markers and techniques and several sample types.

According to the authors, this is the only study to analyze the test’s ability to correctly identify the sex of a fetus across all published reports.

This study provides the most compelling evidence so far of the test’s overall ability to accurately identify fetal sex; it also detects variables that affect the performance of the test.

The study was performed by Diana Bianchi of the Mother Infant Research Institute, Tufts Medical Center, along with Joan Scott and Stephanie Devaney, formerly of the Genetics & Public Policy Center at the Berman Institute of Bioethics at Johns Hopkins University, and Glenn Palomaki of the Women & Infants Hospital.

The researchers found fetal DNA testing in maternal blood had an accuracy rate of 98.6% for female fetuses and 95.4% for males – across 68 published data sets.

They analyzed the following test characteristics:

  • DNA amplification method
  • Gestational age
  • Sample type
  • Y chromosome sequence
  • Year of publication

They found that gestational age and amplification method had the most effect on the performance of the test.

Tests carried out when the fetus was at least 20 weeks old, using a quantitative DNA amplification method (RTQ-PCR) performed the best. However, the authors stressed, reliable tests can be done with good accuracy at 7+ weeks’ gestation. Urine tests, as well as those done before 7 weeks were unreliable.

Some internet sites offer 99% accurate non-invasive fetal sex tests as early as five weeks directly to consumers. Although not yet used in clinical practice in the USA, it is more widely offered in Europe.

The authors wrote:

“Clinical practices that adopt this test to identify fetuses at risk for a sex-specific condition should develop a protocol to validate the presence of fetal DNA in cases of negative results (the absence of Y chromosome). This will ensure that a negative result correlates with the presence of a female fetus and is not simply due to the absence of detectable DNA.”

Written by Christian Nordqvist