A drug that doctors prescribe to prevent mother-to-child transmission of HIV may have small but significant effect on the unborn baby's development, findings of a new study show.

In the new study published in the journal AIDS, researchers from Harvard University have found that the antiretroviral drug atazanavir could affect the development of the infant when taken by HIV-infected mothers during pregnancy.

Atazanavir belongs to a class of drug called protease inhibitor and is used to treat HIV patients as well as lower their odds of transmitting the virus.

The findings showed that the developmental effects on the children occur regardless if the child is born without acquiring HIV infection.

For the new research, which involved 167 women who received atazanavir during pregnancy and 750 who did not, researchers compared the effect of the drug on the women's babies based on developmental baselines once their children turn 1 year old.

The results showed that the babies who were born to mothers who were into antiretroviral regimens that contain atazanavir had lower language and social-emotional development scores compared to children who were not exposed to the drug during fetal development.

The "absolute differences" between the scores of children in the two groups are notably small. In children born to mothers who took atazanavir, their language score was around three points lower than average. Their social emotional score, on the other hand, was five points lower than average.

The effect on social-emotional development was only found in children who started taking the drug during the second to third trimester of pregnancy but language development of the infants were influenced regardless of trimester exposure.

The researchers said that small statistical differences do not have huge clinical implications but these are worth being looked at since they add another risk to a number of biological and socio-environmental risk factors that children of HIV-infected mothers are exposed to.

"In utero exposure to atazanavir-containing regimens compared to non-atazanavir-containing regimens may adversely affect language and social-emotional development in PHEU infants during the first year of life, but the absolute difference is small," wrote Ellen Caniglia, from Harvard T.J. Chan School of Public Health, and colleagues in their study.