A blunder over screening tests at the Norfolk and Norwich University Hospital meant almost 180 pregnant women were given the wrong information about the chances of their children being born with Down's syndrome.

An investigation has been launched and the hospital has apologised unreservedly for the mistake, which meant 176 women were given an inaccurate risk rating.

A dozen women who were at high risk of having a baby with Down's syndrome were mistakenly told they were at low risk, while 164 who were actually at low risk were told the chances were high.

Some of the women have yet to have their babies, but hospital bosses stressed none had chosen to terminate their pregnancy as a result of the information they were given, while, so far, none have had children with Down's syndrome.

Since the mistake was spotted all the women have been given the correct risk level and offered counselling with senior midwives and foetal medicine consultants either on the telephone or by face- to-face meetings.

The mistake was made after a member of staff at the hospital entered the wrong information into the machine which calculates the risks, based on what is known as the first trimester combined screening test for Down's syndrome.

At that point, usually at about 10 to 13 weeks into pregnancy, the results of blood tests and an ultrasound to check the thickness of the baby's neck (a build-up of fluid there is an indicator of a child with Down's syndrome) are combined with information about the expectant woman such as her age, weight, diet and general health.

That enables a calculation of the level of risk of pregnancy resulting in a baby with Down's syndrome –and it picks up 85pc of babies who have the condition.

The test is not compulsory and 60pc of pregnant women take the test. The N&N laboratory also calculates the risk for women tested at the James Paget University Hospital in Gorleston and the Queen Elizabeth Hospital in King's Lynn.

It was discovered that the wrong information had been entered in July last year as part of the six-monthly quality assurance undertaken by the Down's Syndrome Screening Quality Assurance Support Service (DQASS).

Hospital bosses said they corrected it as soon as they became aware of it but it meant they had to check again the 2,247 tests which had been done from July 2009 through to October 15, which was when it was found the results had been skewed.

Chief executive Anna Dugdale said the hospital, at which more than 5,800 babies have been born this year, accepted full responsibility for the error and had apologised unreservedly to all the women who had been affected.

She said: 'As a hospital we are absolutely committed to providing the care we would want for our families to all of our patients.

'We do not always get it right and it is my firm belief than when this happens we should be open and honest with our patients and learn from our mistakes.

'We have already taken steps to make sure that the learning from this will benefit both our future patients and patients more widely within the NHS.'

The investigation which the hospital launched has already established what went wrong, but will now continue to establish why the wrong information was entered, which could lead to disciplinary action against those responsible.

The low-risk women who were told they were high risk were informed they had a higher than 1 in 150 chance that their baby would be born with Down's syndrome.

Of the 12 high-risk women who were told they were low-risk - basically that their chances of having a Down's syndrome child were lower than 1 in 150 - five of them decided to have a further diagnostic test called an amniocentesis.

That test involves using putting a fine needle through the wall of the womb to remove a small amount of the amniotic fluid around the unborn baby and can identify for certain whether a baby has Down's Syndrome.

Of the 164 who thought they were at high risk, 119 of them decided to have a diagnostic test such as amniocentesis and 45 did not.

The test for amniocentesis creates a 1 in 100 chance of leading to a miscarriage, while there is also a small risk (less than 1 in 1000) that the procedure will cause a serious infection.

So far, none of the women who are still pregnant have discovered they are expecting a child with Down's syndrome.

• Were you one of the women affected? Call reporter Dan Grimmer on 01603 772375 or email dan.grimmer@archant.co.uk