One in 25 people carry the cystic fibrosis gene
Blood taken from a pregnant woman may reveal if her baby has a wide range of genetic diseases, researchers claim.
A Chinese University of Hong Kong team said the technique could identify cystic fibrosis, beta-thalassemia and sickle cell disease.
The only checks currently available carry a high risk of miscarriage.
The test works by checking foetal DNA in the mother's blood against her own, the Proceedings of the National Academy of Sciences reported.
People have two copies of every gene, one from their father and one from their mother. When they conceive a child they pass on one of these gene copies.
However, couples trying for a baby may be oblivious to a hidden threat posed by "recessive" genetic disorders.
An estimated one in 25 people in the UK carries one copy of the gene for cystic fibrosis, but only people who have two "bad" copies will actually develop the disease.
However, if both parents carry a single disease gene, there is a 25% chance that a child they produce will inherit both, and become ill.
Parents who suspect or know they carry one or two copies of the disease gene can use IVF techniques to produce embryos, and test the genetic make-up of each one before implantation to check it is free of the condition.
The discovery that DNA from the unborn child can be found in the mother's plasma - the part of the blood once cells have been removed - opened new possibilities for testing.
About 10% to 15% of the DNA in the plasma comes from the baby, and the rest belongs to the mother.
Scientists can look for defective DNA sequences passed from the father this way, but it is far more difficult to check for faulty sequences passed on by the mother - as they are identical to the "background noise" - the faulty sequences in the mother's DNA.
The Hong Kong team may have overcome this, by devising a method to check for minute differences in the amount of faulty DNA carried in the mother's plasma compared to the sample derived from the unborn baby.
In a healthy non-pregnant woman carrying the disease, with one normal and one faulty gene, exactly half the DNA sequences will be faulty and half non-faulty, mirroring her genetic makeup.
If she is pregnant, and the baby also has inherited the same genetic make-up, these proportions will remain the same.
However, if the child has two copies, and is destined to develop the disease, the numbers of faulty genes in the mixture will be very slightly higher, and by using digital technology to count this, an accurate assessment can be made, say the scientists.
Professor Dennis Lo, who led the study, said that the advance meant that the test could, in theory, be used by doctors.
"This addresses a problem that has been puzzling investigators in the field of non-invasive prenatal diagnosis over the past 10 years," he said.
A spokesman for the Cystic Fibrosis Trust said: "We are very interested to hear about this new research, which in principle should make it easier and less risky to test a foetus for cystic fibrosis.
"At present parents who wish to know if their foetus has cystic fibrosis can have chorionic villus sampling or amniocentesis.
"As this carries a risk of miscarriage, it is usually only recommended for those who know they are carriers of the gene, often those who already have one child with cystic fibrosis."