Such dilemmas are typical of those faced by mothers who, with the advance of modern medicine, are being asked to make more decisions about the fate of their babies.

William So Wai-ki, a spokesman for the College of Obstetricians and Gynaecologists, said the number of prenatal screenings was growing in Hong Kong as more tests became available.

Screenings have accelerated in tandem with a trend for couples to have only one child and at a later age, when risks of defects are higher.

'From the ethical point of view, the increasing number of prenatal screenings is moving towards the so called 'perfect baby',' Dr So said. 'Doctors always have to face an ethical question - should they offer all tests to everyone who walks into their clinic?'

He said the profession believed in a philosophy that prenatal screenings should only be applied on 'debilitating and rather common diseases'.

In Hong Kong, the three most common prenatal screening tests are for Down's syndrome, the inherited blood disorder thalassemia, that is treated with a lifetime of transfusions, and the Aids virus HIV.

'We think Down's is a reasonably debilitating disease and rather a common risk as the maternal age is getting higher and higher, that's why we agree prenatal screening should be done,' Dr So said.

Doctors will also give priority to the well-being of the parents rather than the child.

'Of course, it does not mean we won't consider the interest of the baby at all,' Dr So said. 'But in most situations, parents will find it difficult to look after a Down's baby.

'Some parents' concern is not about the present but the future. Who will take care of the child when they get old? So we think it is acceptable to screen for Down's. Of course, the pro-life activists may not agree with us.'

Down's syndrome, caused by a chromosome abnormality, occurs about once in every 700 births. Affected children have different degrees of mental disability. They may also develop dementia near the age of 40.

Major physical abnormalities are prevalent in the heart, brain and other organs, with about half also suffering from congenital cardiovascular diseases.

A Hospital Authority spokesman said the current practice in public hospitals was to provide prenatal diagnostic testing for Down's syndrome in pregnant women aged 35 or above, and for those who were otherwise classified as high-risk.

Three clinical tests are available: integrated ultrasound and blood tests; a blood test alone; and amniocentesis - in which a needle is inserted into the womb to draw out fluid. The latter carries a one in 200 chance of miscarriage.

Doctors estimate the risk of Down's syndrome among babies born to mothers aged 35 or above is one in 400 and it increases to one per cent for those who give birth at the age of 40 or above. For mothers under 35, the chance falls to 0.1 per cent.

Sometimes the decision on whether a screening test should be done is not purely medical, but political. For example, although HIV is not a common disease in Hong Kong, the government's policy on Aids prevention prompted the introduction of HIV screening for pregnant women.

Since September 2001, the HIV test has been offered free to women having their first prenatal check-up, in addition to standard tests for hepatitis B, syphilis and rubella.

Dr So, a former consultant at the Queen Mary Hospital and now a private practitioner, said that for years he and his former colleagues at the public hospital had urged the Hospital Authority to introduce universal blood tests for Down's syndrome among all pregnant women, regardless of age.

But the authority management has strongly opposed the proposal.

'The management believes that it is in conflict with the image of the authority,' Dr So said. 'The authority believes that as it always encourages the public to accept people with disabilities, it would be a conflict if it tries to select a group of handicapped people.

'We cannot change the reality that if a baby is Down's, all we can offer is abortion,' Dr So said.

Dr So warned that using the maternal age as one of the criteria for screening Down's syndrome was outdated.

'We know that the risk increases with the maternal age,' he said. 'A screening test aims to identify the five per cent of the highest risk. But it depends on the actual age distribution of the population. For example, if the population is ageing, the cut-off point will be above 38 years old.'

Dr So said that according to the current screening practice - using the maternal age of 35 as the cut-off point - only 30 per cent of Down's babies would be detected. But if blood tests were made available to all pregnant women, followed by an amniocentesis offered to those defined as high-risk mothers, more than 60 per cent of all Down's babies would be identified before birth.

Private doctor Louis Shih Tai-cho knows what it means to fall between the cracks in the health system.

Dr Shih, a dermatologist, has a 17-year-old daughter with the disease. While living in Canada, the doctor and his wife requested an amniocentesis because one of their relatives had previously given birth to a Down's baby.

But the request was turned down because Dr Shih's wife was then only 34 years old, one year younger than the cut-off age for the test offered by the public health-care system.

Dr Shih said they never regretted having their daughter, Evelyn. 'My daughter enriches our lives. We did not know in the past that a Down's baby could be that adorable.'

But in hindsight, he would still prefer to have the test done. 'Probably we would still have kept the baby even if we knew the truth, but at least we have the right to know,' he said.

Dr Shih, who is an active member of the Down's Syndrome Association, said he had come across cases in which couples divorced after giving birth to a Down's baby. 'We should not impose our mentality on other couples,' he said. 'A child born with special needs would inevitably cause stress to a marital relationship.'

Medical experts are busy developing new and safer tests for Down's syndrome. For example, a University of Hong Kong research team is developing a technique for early diagnosis of the disease by measuring the noses of unborn babies.

Overseas research found that a foetus with the condition tended to develop a nose bridge about half the length of those in normal babies, when the mother is between 16 and 20 weeks pregnant.

Two-hundred unborn babies have been studied since the research was launched last year and the team hopes to take another 800 samples to obtain a more accurate standard. The research is expected to take another two years.

Dr So said that while parents' desire for a healthy baby was moving towards trying to create a 'perfect baby', the future trend of prenatal screening would also move closer to pre-implantation diagnosis.

The term meant doctors conducting in-vitro fertilisation would ensure an embryo was healthy before implanting it into the mother's womb.

Dr So said that couples who both carried the genes of thalassemia were being offered this test. 'With the pre-implantation diagnosis, the mother will not have to worry that her baby will be thalassemia major. It avoids having another abortion,' he said.