WOMEN'S HEALTH
February 15, 2010

Blood Test Reveals Fetal Sex

A simple blood test may replace amniocentesis as the best means for determining a baby's sex early in utero

A new maternal blood test, developed in the Netherlands, is highly accurate at determining fetal sex early in pregnancy.

Amniocentesis...involves inserting a needle into the womb and removing some fluid. This procedure carries a small amount of risk, risk that is unnecessary if a blood test will give the same information.

The test relies on finding fetal DNA in the mother's blood. It was 100% accurate in determining the sex of the fetus when DNA was found and about 95% accurate overall. It tests for two genes found only on the Y chromosome. When these genes are found, the fetus is male. If DNA is detected but these genes are not, the fetus is female. In about 5% of the cases, insufficient DNA was found to make any sex determination. The test was done as early as seven weeks into pregnancy.

Normally, the sex of a fetus can be determined early in the second trimester by ultrasound. If there is a medical need to determine this earlier, a procedure called amniocentesis is used. This involves inserting a needle into the womb and removing some fluid. This procedure carries a small amount of risk, risk that is unnecessary if a blood test will give the same information.

There are many conditions where early sex determination is medically important. Congenital adrenal hyperplasia (CAH) is a genetic disorder which causes girls to develop many male physical characteristics. Symptoms can be prevented or minimized if the mother starts taking the steroid dexamethasone very early in pregnancy. There is no need for this if the fetus is male. In the study, 27 women were able to stop taking steroids when the test showed the fetus to be male.

In addition, there are many X−linked genetic conditions, such as hemophilia, which occur almost entirely in boys. In cases where a mother is known to carry a genetic trait for such a condition, further invasive testing is generally performed during pregnancy. If the fetus can be shown to be female, these invasive procedures need not be done.

The study looked at 201 pregnant women between 2003 and 2009, many of whom were tested because of the risk of an X−linked disorder. The sex of 189 fetuses was determined accurately. In 10 cases, no fetal DNA was found and no sex determination was made. These 10 cases all turned out to be female. The other two cases were early miscarriages.

The study does not discuss costs or when the test might become generally available. Its results were published in the January 2010 issue of Obstetrics and Gynecology.

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