Doctors may soon be able to diagnose inherited diseases such as cystic fibrosis, thalassemia and sickle cell anemia in fetuses by simply testing a blood sample taken from the mother.

Until now, prenatal diagnoses of such disorders have been possible only through invasive procedures like amniocentesis, which carry a risk of fetal miscarriage.

Amniocentesis is the extraction of a small amount of fluid from the sac surrounding a developing fetus.

But scientists in Hong Kong and Thailand may have found a way to diagnose in fetuses such "monogenic" diseases, which are caused by a single error in a single gene in the human DNA.

"Such diseases can be diagnosed by a simple blood test (taken from the mother) ... and by counting the relative ratio of the mutant genes against the normal genes," said lead researcher Dennis Lo at the Chinese University of Hong Kong.

This is only possible because fetal DNA circulates in maternal blood, a discovery Lo and his colleagues made several years ago.

Many scientists have since been trying to find the best way to differentiate fetal DNA from maternal DNA, before they can even get down to looking for any anomalies in the fetal DNA. But these efforts have not met with much success.

In an article published in the Proceedings of the National Academy of Sciences, Lo and his colleagues said they had devised a counting system that could "bring non-invasive prenatal diagnosis of monogenic diseases closer to reality."

Using highly precise digital blood testing technology, both mutant and normal DNA sequences are counted in maternal plasma and that is then used to calculate the number of mutant genes inherited by the fetus and to determine the probability of the fetus developing any monogenic disease.

Lo, however, noted that the accuracy of this method would depend on the concentration of fetal DNA in maternal blood.

Thalassemia is a blood related genetic disease that can result in reduced fertility or even infertility. Early treatment can improve the quality of life of patients.

Cystic fibrosis affects the respiratory, digestive and reproductive systems and can lead to fatal lung infections.