A new ultrasound test can pick up 20% more Down's syndrome foetuses than traditional analysis, according to scientists.

Doctors from a London research group assessed 96,127 pregnant women at 22 clinics around the country.

They singled out which women were most likely to develop Down's Syndrome, based on their age and on an ultrasound scan showing the thickness of skin at the back of the foetus' neck.

Swelling of the back of the neck in foetuses after 10 to 14 weeks of pregnancy is thought to be a pointer to Down's syndrome. Older women run more of a risk of giving birth to Down's babies.

The scan was more than 82% accurate, compared with a 60% accuracy rate for current tests involving selection of risk by the mother's age and an analysis of her blood and lymph make-up.

Screening by age alone picks up 30% of cases.

Needle tests

Scientists from the Foetal Medicine Foundation First Trimester Screening Group in London published their findings in this week's Lancet.

Women identified as being at high risk of giving birth to a Down's syndrome baby have to undergo further tests, involving needle probes into the mother's womb.

Only 5% of cases are found to be high risk. The FMF doctors said the ultrasound method of risk assessment would still involve around 30 invasive tests to identify one affected foetus.

One in 1,000 babies are born with the extra chromosome that results in Down's Syndrome. There are three types of the syndrome, including genetically inherited Down's syndrome.

The abilities of Down's children and adults varies according to the severity of their disability.