The test is important, write the authors of the study, because in some cases, there are medical reasons to determine foetal sex earlier. That has traditionally meant invasive tests, like amniocentesis, which carry a small risk of miscarriage.
Consequently, maternal blood tests that pick up certain markers of foetal sex have been developed and put into use in recent years. So far, research has shown the accuracy of these tests to vary widely, depending on the methods used.
In this latest study, published in the journal Obstetrics & Gynecology, researchers in the Netherlands found that the testing method used at their centre was 100% accurate in determining foetal sex among nearly 200 pregnant women.
Part of what's new about the findings is that they show the effectiveness of blood testing as it is done in routine practice - and not just in the research setting, Dr Ellen van der Schoot, of Sanquin Research Amsterdam, told Reuters Health in an email. The study does not discuss costs, nor when the test might be available to the general public.
Still, the findings, according to van der Schoot and her colleagues, support using the tests in cases where foetal sex is important in detecting or managing certain inherited medical conditions.
For example, congenital adrenal hyperplasia (CAH) is a genetic disorder that causes girls to develop abnormal external genitalia and male-like characteristics like a deep voice and excessive body hair.
It is possible, however, to treat the disorder with the steroid dexamethasone as soon as pregnancy is established, so knowing the foetal sex sooner is better than later.
Similarly, foetal sex is key in genetic disorders linked to abnormalities in the X chromosome. These disorders - such as hemophilia and Duchenne/Becker muscular dystrophy - are almost always seen in boys rather than girls, because boys inherit only one X chromosome, from the mother. (Girls inherit an X chromosome from each parent.)
In cases where a mother is known to carry an X-linked genetic defect, blood testing for foetal sex tells doctors whether further, invasive testing for the particular genetic disorder should be done. If the foetus is female, invasive tests can be avoided.
In the current study, van der Schoot and her colleagues looked at 201 pregnant women who had blood testing at their lab between 2003 and 2009. The test, done as early as the seventh week of pregnancy, determines foetal sex by looking for two genes found on the Y sex chromosome.
Only men carry the Y chromosome, so when these genes were found in a pregnant woman's blood sample, the foetus was assumed to be male. When the test did not detect the genes, the mother's blood was analyzed further to confirm that certain other foetal DNA was present; with that confirmation, the researchers concluded that the foetus was female.
Of the 201 women in this study, blood tests gave conclusive results to 189. In each case, that result turned out to be correct.
The test results also made a significant difference in how the pregnancies were managed from then on, according to van der Schoot and her colleagues. Among the 156 women who underwent testing because of the risk of an X-linked disorder, the results allowed 41% to avoid further, invasive procedures to test for the disorder.
In addition, 27 women who'd been taking steroids for possible CAH were able to stop, as the foetus was found to be male.
According to the researchers, the reliability of the blood tests means that invasive procedures are no longer necessary for determining foetal sex early in pregnancy.
Since the introduction of such blood testing, there have been ethical concerns about couples seeking testing solely to find out the sex of the foetus early on - and possibly ending the pregnancy based on that information.
"These concerns are relevant," van der Schoot said, "and we agree that the test should be applied carefully in a clinical setting upon medical indication."
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