Specialists advise HIV-positive women who become pregnant to continue taking treatments, although they also question the advisability of becoming pregnant while on such drugs.

However, doctors say the treatments are now so good that women taking them will feel well enough to have a child.

They are calling for wider studies to establish exactly what the risks are.

Cell fault

Research into the problem performed by Dr St�phane Blanche of the Necker Hospital in Paris is reported in New Scientist magazine.

Dr Blanche, who is preparing a scientific paper on the subject, said: "All obstetricians and gynaecologists in France have been informed about our findings."

The condition to which Dr Blanche draws attention is one caused by faults in the energy-producing elements in cells - known as mitochondria.

Gareth Tudor-Williams is a paediatric Aids specialist at Imperial College School of Medicine in London.

"This condition is an extraordinarily rare mitochondrial disorder that you might expect to see in only 1 in 10 000 or 1 in 100 000 births," he told the magazine.

"If a link is confirmed we will need to do a very careful search of the data worldwide."

Benefits outweigh risks

Concerns have been raised further by a second study suggesting that the drugs may increase in the risk of cancer.

However, doctors say that the benefits of taking the drugs far outweigh the risks, and pregnant women should continue on their current treatments.

The chance of passing the infection on to a child if medication is not taken is about 14% if a woman does not breast feed and about 25% if she does.

Some studies suggest modern medication can reduce this to as low as 1%.

Andrew Ridley, director of the Terence Higgins Trust (THT), told BBC News Online the finding was of concern and said their should be a further examination of current data until new research can get started.

"However, people with HIV need to bear in mind the study's too small to be conclusive.

"Also, the clinical benefits of taking therapy during pregnancy remains true in terms of preventing HIV transmission."