Appropriate treatment can prevent risk of HIV transmission from mother to child, a UK study shows.
Data on 5,151 HIV pregnancies in the UK and Ireland between 2000 and 2006 found an infant infection rate of just 1.2% where preventative steps were taken.
In the mid-1990s, before effective drug therapy became available, the infant infection rate was over 20%.
University College London led the Aids Online study, BBC reports.
The researchers said it was the first time such low rates of infection had been observed at a population level.
Most HIV positive women in the UK now take a combination of antiretroviral therapy (ART) drugs during pregnancy.
A caesarean section delivery reduces the risk of infection to the child - but the latest study showed that in many cases the drugs are so effective that a normal delivery is possible.
Transmission rates for women on ART for at least the last 14 days of pregnancy were 0.8% - regardless of the type of delivery.
The researchers said the key to success was that most women in the UK and Ireland now accepted antenatal testing for HIV.
The introduction of routine screening saw the estimated proportion of infected women diagnosed before delivery rise from about 70% in 200 to about 95% in 2005.
However, a 2006 report from the World Health Organization found that access to ART drugs is severely limited in developing countries.
The authors found that under 10% of pregnant women with HIV in these countries had access to the drugs.
As a result, they calculated, about 1,800 babies were born with HIV each day because their mothers did not get the drugs they need.
Lead researcher Claire Townsend said that for women with access to drugs, the findings were "greatly encouraging".
She said: "They demonstrate that if women are tested for HIV early enough in pregnancy for ART to be initiated, the risk of infection to their baby is very low indeed.
"This emphasizes the importance of achieving and maintaining a high uptake of antenatal HIV testing on a national scale."
Lisa Power, of the HIV charity Terrence Higgins Trust, said: "With the right treatment and relevant support, the vast majority of women living with HIV can have healthy uninfected children.
"This is why testing for HIV in pregnancy is so important and why treatment for pregnant women living with HIV in the UK should always be free, whatever their immigration status."