New Down syndrome non-invasive test for ‘mums-to-be’ on the horizon

By Dr. Ananya Mandal, MDJan 12 2011

According to experts a simple DNA test for Down syndrome could save nearly all pregnant women from invasive tests like amniocentesis. Usually 3% to 5% of pregnant women in the UK or some 30,000 women have to undergo invasive tests like amniocentesis that increases the risk of miscarriage to 1%. The new DNA blood test could bring this down to 0.1%, according to a study in the British Medical Journal.

Faced with risk of miscarrying many women choose not to go for a diagnostic test. Amniocentesis involves having a needle inserted into their womb to draw off a sample of placenta cells or some of the fluid that bathes the baby. It is recommended for women who are at risk of giving birth to a baby with Down syndrome. The new non-invasive DNA blood test could offer another option.

Technically humans have 23 pairs of chromosomes. Babies with Down syndrome have an extra copy of chromosome 21, causing physical and intellectual impairments. The major developmental disorder occurs in around one in every 800 live births. In 1997, Dr. Dennis Lo and his co-researchers from The Chinese University of Hong Kong discovered that during pregnancy a foetus releases its DNA into the mother’s blood plasma. Now they know that the entire foetal genome is present in the mother’s blood. Since DNA can cross the placenta from the baby to the mother, the blood test in the mother can look for this extra chromosome. Scientists believe it should be rolled out as a screening test in the future based on their findings.

This study involved 753 pregnant women in Hong Kong, the UK and the Netherlands and showed that it could bring the number of invasive tests down significantly, by about 98%. At present the NHS screening has a “false positive” rate of about 5%, meaning 5% will be told they are carrying a baby with Down’s when they are not. If these women were given the DNA blood test instead, almost all invasive procedures could be avoided, according to the researchers. Of the 30,000 women who undergo amniocentesis, with the new test only 4,000 would need it.

Lead researcher Professor Kypros Nicolaides of King’s College London along with colleagues from The Chinese University of Hong Kong, said the test would be welcomed by many women. He said, “Some women, understandably, are fearful of invasive tests… This extra screen is non-invasive and would save many from needing further investigation…Our study shows it is feasible to use in clinical practice.” He added however that the test was still too expensive and needed further study before it could be rolled out to be used routinely - something that could take 10 years. The ultimate goal is to make it 100% accurate so that invasive tests could be dispensed with completely he said.

Professor Lyn Chitty of University College London an independent observer who is also working with DNA blood tests for Down’s said, “I suspect there are many women who would welcome such a test and it may lead to a lot more women accepting the offer for screening.” But, she added that pre test counselling of women is still important. “As yet, the results are not accurate enough to inform important decisions, like whether to continue the pregnancy,” she said.