A new non-invasive blood test can reliably detect whether or not an unborn baby has Down's syndrome during the first trimester of pregnancy, researchers have said.
The test, which analyses a baby's DNA present in its mother's blood, could be an improvement on current screening methods, according to the study.
Down's syndrome, and other chromosomal disorders characterised by an additional chromosome, is currently tested for between the 11th and 13th weeks of a pregnancy, using an ultrasound screen and a hormonal analysis of the pregnant woman's blood.
The only definitive test involves chorionic villus sampling and amniocentesis but that carries a risk of miscarriage.
A study, published in the Ultrasound in Obstetrics & Gynecology journal involved mothers-to-be at King's College Hospital, London.
It is the first to look at the feasibility of screening for the chromosomal disorders - known as trisomies - using foetal cell free DNA (cfDNA) from a pregnant woman's blood.
The tests, carried out at the 10-week stage on 1,005 pregnancies, were found to be more sensitive at detecting Down's syndrome and returned fewer "false positive" results than the combined tests usually carried out between the 11th and 13th weeks. The cfDNA test returned false positive rates of 0.1% compared to 3.4% for the combined tests.
The study's authors said: "This study has shown that the main advantage of cfDNA testing, compared with the combined test, is the substantial reduction in false positive rate. Another major advantage of cfDNA testing 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."
They also found that if cfDNA testing is carried out after a combined test at 11 to 13 weeks, 98% cases of Down's syndrome could be detected. Invasive testing was then needed for confirmation in less than 0.5% of cases.
The authors added: "Screening for trisomy 21 by cfDNA testing contingent on the results of an expanded combined test would retain the advantages of the current method of screening, but with a simultaneous major increase in detection rate and decrease in the rate of invasive testing."