The Ministry of Health is urgently reviewing tests of unborn babies for Down syndrome after women were found to be putting their babies' lives at unnecessary risk.

An expert report says too many woman are undergoing invasive tests which have a 0.5 to 1 per cent chance of causing miscarriage.

Professor Peter Stone has told the ministry's National Screening Unit the screening does not meet safety requirements and must be improved.

The unit says a panel will review screening with an external advisory group, and will report to Health Minister Pete Hodgson early next year.

Amniocentesis is done on 5 per cent of pregnant women.

"Screening programmes have safety and quality requirements. These are not being met by the current approaches to Down syndrome screening," says Professor Stone's report. "More normal pregnancies may be adversely affected by amniocentesis than the numbers of Down syndrome fetuses detected."

The ministry says concerns have been raised that the invasive tests are being done on some women who may not have an increased chance of having a child with Down syndrome and who have not been told of the risk of miscarriage associated with the tests.

The review was about unnecessary harm to women and fetuses, the ministry said, " ... this is not about reducing the numbers of babies born with Down syndrome."

The ministry has written to thousands of health workers, including doctors, midwives and those involved in ultrasound scanning, telling them to do better in respect of Down screening and the information they provide.

"Maternal age is not a sufficient screening test by itself," the letter says.

The screening unit's manager, Karen Mitchell, said it was important women were informed before testing.

"Some women may not choose to have the invasive procedure because of the risk and they shouldn't be pressured to do so."

Health workers have been told that each pregnant woman should be offered a test to assess her individual chance of having a Down child.

The chance of having a baby with Down syndrome, a chromosome abnormality that results in intellectual disability and an increased risk of some physical problems including heart disease, rises steeply after 35.

New Zealand's incidence of Down syndrome rose from 0.87 per 1000 live births in 1996 to 1.48 (80 Down babies) in 2002.

An ultrasound scan about the 11th week of pregnancy is the main screening method. Women considered at risk on the basis of their age and the scan are referred for a diagnostic test.

Down Syndrome Association executive officer Zandra Vaccarino said comprehensive information needed to be given to women considering screening. What they were told varied greatly around the country - and in some cases was outdated.

THE TESTS

* New Zealand's main Down screening test is ultrasound scanning, which measures the thickness of fluid space at the back of the fetus' neck.

* For those at greater risk, the main diagnostic test is amniocentesis, where a sample of the fluid around the fetus is obtained through a needle inserted into the uterus.

* 6300 women a year have the diagnostic tests.