Parents who want to know the sex of their baby before it is born usually find out through ultrasound done in the second trimester.

However, there is a blood test that can be done early in pregnancy which is highly accurate in determining the sex of the foetus, a new study has found.

The test is important, the authors of the study wrote, because in some cases there are medical reasons which make it necessary 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 varies 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 per cent accurate in determining foetal sex in nearly 200 pregnant women.

Part of what is new about the findings is that they show the effectiveness of blood testing as it is done routinely - and not just in the research setting, Dr Ellen van der Schoot of Sanquin Research Amsterdam told Reuters Health. The study does not discuss costs, nor when the test might be available to the public.

Still, the findings, according to Dr 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.

In the current study, Dr van der Schoot and her colleagues looked at 201 pregnant women who had their blood tested at their lab between 2003 and last year.

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.

Of the 201 women in this study, blood tests gave conclusive results for 189 women.

The test results also made a significant difference to how the pregnancies were managed from then on, said Dr van der Schoot and her colleagues.

Among the 156 women who underwent testing because of the risk of an X-chromosome-related disorder, the results allowed 41 per cent to avoid further, invasive procedures to test for the disorder.

The researchers said the reliability of the blood tests means that invasive procedures are no longer necessary for determining foetal sex early in pregnancy.

Since their introduction, 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," Dr van der Schoot said, 'and we agree that the test should be applied carefully in a clinical setting upon medical indication."

The test determines foetal sex by looking for 2 genes found on the Y sex chromosome.

This article was first published in Mind Your Body, The Straits Times.