Tests that measure DNA in a pregnant woman's blood work well for telling the sex of a baby after seven weeks' gestation without posing danger to the fetus, a US study said Tuesday.

The meta-analysis of previous studies on the topic suggests that using cell-free fetal DNA from the mother's blood is more accurate than a urine test or sonogram and is safer than amniocentesis.

It can also be done earlier than an ultrasound, which is usually accurate from 11 to 14 weeks, or amniocentesis, which samples fluid from the sac surrounding the fetus and carries a small risk of miscarriage.

"The availability of a reliable noninvasive alternative to determine fetal sex would reduce unintended fetal losses and would presumably be welcomed by pregnant women carrying fetuses at risk for disorders," said the study in the Journal of the American Medical Association.

The review included 57 studies, representing 3,524 male-bearing pregnancies and 3,017 female-bearing pregnancies, and found an accuracy rate of 95 to nearly 99 percent.

The technology to detect cell-free circulating Y-chromosome DNA sequences in the plasma of pregnant women has been around since 1997.

Many countries in Europe, including France, Spain, Britain and the Netherlands, routinely offer the test to pregnant women.

Some companies also sell the tests to consumers over the Internet.