Washington: Scientists have claimed that a using a certain type of injectable birth control could fairly increase the risk of becoming infected with HIV.

A large meta-analysis of 12 studies in sub-Saharan Africa found that women who used the contraceptive, depot medroxyprogesterone acetate, which is sold under the brand name Depo-Provera, had a moderately increased risk of becoming infected with HIV.

The findings included data from 39,500 women. The researchers selected the studies based upon methodological rigor, such as whether they accounted for the use of condoms.

In addition to Depo-Provera, the studies also examined other commonly prescribed forms of hormonal contraception, such as the injectable norethisterone oenanthate (sold as NET-EN), combined oral contraceptives and progestin-only pills. The other birth control methods did not appear to increase HIV infection risk for women in the general population.

Study's lead author Lauren Ralph of UC Berkeley, said that the results had potentially broad implications because hormonal contraceptives remain popular for women worldwide.

Approximately 144 million women worldwide use hormonal contraception, and of those about 41 million women use injectable forms of birth control instead of the pill.

The study found that women who used depot medroxyprogesterone acetate had a moderate, 40 percent increased risk of acquiring HIV compared with women using non-hormonal methods and those not practicing birth control. The increased risk was slightly lower, 31 percent, among the studies done in women in the general population.

It remains unclear why the increased risk was seen among those using Depo-Provera but not the other forms of hormonal contraception, the authors said. One possibility may be that birth control with higher levels of progestin, the synthetic form of the natural hormone progesterone, changed the vaginal lining or altered local immunity, increasing the risk for HIV infection, though the researchers emphasized that this study did not examine the physiological effects of the different contraceptive methods and more research on potential underlying biologic mechanisms is needed.

The researchers cautioned that the increased HIV infection risk needs to be considered in the context of the risks associated with not using birth control.

The researchers noted that the results highlight the need for more studies among high-risk populations. Among the 12 studies analyzed, only two included sex workers or women with HIV-positive partners.

The study is published in The Lancet.