Treatment used in poor countries to prevent HIV passing from mothers to babies may have a serious drawback.
Sophisticated drugs are denied to many Africans
Research suggests it may reduce the drug's effectiveness when it is needed later by the women themselves.
In some impoverished, high-risk areas, HIV positive pregnant women are given a one-off shot of the drug nevirapine.
However, research presented at the Annual Retrovirus Conference in San Francisco suggests this is enough for HIV to build resistance to the drug.
Nevirapine is used because is cheap, and has been shown to cut by half the risk of a pregnant woman passing HIV on to her child.
However, one study, conducted in South Africa, found that 39% of HIV positive women who get nevirapine during pregnancy go on to harbour virus that is resistant to the drug.
A second study, done in Thailand, found mothers who eventually needed drug treatment for their infection were much less likely than usual to respond to it.
Lack of alternative
Kate Carr is president of the Elizabeth Glaser Pediatric Aids Foundation, the largest provider of treatment to prevent mothers passing on HIV to their babies in Africa and other poor parts of the world.
She said: "The sad reality is, we do not have another option in most places, either for mothers or babies."
The World Health Organization estimates that 40% of pregnant women are infected with HIV in some African countries.
In the United States and Europe, pregnant women with HIV are routinely given combinations of powerful AIDS drugs throughout their pregnancy.
Not only does this reduce the risk of passing the virus on at birth to near zero, it also minimises the chance of developing resistance.
But in Africa and other poor areas this approach is not considered practical.
Instead, the WHO recommended giving a single dose of nevirapine when labour starts and then giving a second pill to the baby two days after birth.
About 8% of babies catch the virus after nevirapine, compared with about 20% of those who get no treatment.
Doctors at the conference said several alternatives were under consideration.
Dr Isabelle De Zoysa, WHO director of HIV prevention, said the organisation had recently met to discuss the use of nevirapine.
She said the evidence of problems was far from conclusive. For instance, nobody at present could quantify how much resistance HIV would develop spontaneously.
"It is a safe and very efficacious regimen, and there does not seem to be good enough reason at this time to switch to another regimen which may be less efficacious and lead to more infant infections."