Researchers are reporting that they are now able to determine a foetus’ gender with 100% accuracy at five weeks using a maternal blood test. Previous tests had only been available for women several months into their pregnancies. The findings were reported this week in the publication Obstetrics and Gynecology.

The study was conducted from 2003 to 2009 and included 201 expectant mothers in the Netherlands who were referred by geneticists or gynaecologists advising them on possible genetic diseases. Although the researchers were not able to issue a conclusion in 12 of the cases, in the 189 cases on which they did make a determination, they were correct 189 times.

The technique is non-invasive and is said to be safer than amniocentesis and chorionic villus sampling, both of which carry serious risks and can only be performed later in pregnancy. Since several genetic disorders are linked to the X chromosome, for example, this blood test will allow families with a probability of gene-based disorders to know early whether they have a boy, who will likely carry the disease, like hemophilia, or a girl, who will not.

In addition, if the blood of a woman whose foetus was likely to have a sex-based abnormality confirmed that she’s carrying a boy, invasive testing is warranted. But if she’s carrying a girl, she and her foetus are spared the additional testing.