A non-invasive blood test carried out in the first trimester of pregnancy may be more accurately able to detect Down syndrome, scientists have claimed.

Down syndrome is a chromosomal anomaly caused by an error in cell division that results in an extra copy of the 21st chromosome usually in each cell of the body. This is known as trisomy 21.

This extra genetic material results in the learning and physical characteristics associated with Down syndrome. It is one of the most common known causes of intellectual disability.

Currently, women who wish to assess their risk of having a child with Down syndrome can undergo a combined test between weeks 11 and 13 of their pregnancy. This involves an ultrasound and a blood test.

The blood test checks for abnormal levels of certain hormones and the ultrasound focuses on measuring the nuchal fold of the baby. This is found on the back of the neck and Down syndrome babies tend to have thicker nuchal folds.

Following this, a woman will be given an estimate of how likely she is to have a baby with Down syndrome, for example, one in 1,000 or one in 150. If there is a high risk, she may opt to have one of two tests which can definitely determine if a baby has Down syndrome - chorionic villus sampling, which tests a sample of the placenta or amniocentesis, which tests the amniotic fluid around the baby.

However both of these tests are invasive and carry a risk of miscarriage.

This new blood test appears to be more accurate than the current combined screening method of ultrasound and blood test. It analyses the foetal DNA in the pregnant woman's blood.

The test can also be carried out earlier in the pregnancy.

UK scientists carried out the test on 1,005 pregnant women during their 10th week of pregnancy. They recorded fewer false positive test results compared to the other method and a higher sensitivity for trisomy 21.

"This study has shown that the main advantage of this testing, compared with the combined test, is the substantial reduction in false positive rate. Another major advantage is the reporting of results as very high or very low risk, which makes it easier for parents to decide in favour of or against invasive testing," the scientists from King's College London explained.

The team is planning a further two-year study of the test, involving some 20,000 women.

Details of these findings are published in the journal, Ultrasound in Obstetrics & Gynecology.

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