CHICAGO (CNN) -- Giving mothers and their newborn babies standard AIDS medications for just a week at childbirth cuts the risk of HIV transmission from mother to child by more than a third, researchers report.

A study presented Monday at the 6th Conference on Retroviruses and Opportunistic Infections said a combined treatment of AZT and 3TC, both made by Glaxo Wellcome, is relatively effective in blocking HIV-infected mothers from passing the virus on to their child during childbirth or breast feeding.

Doctors have long known that AZT can cut transmission risk in pregnant women. However, treatment is usually given for weeks or months after childbirth, an expensive option not available to many women in poor countries.

Although long-term treatment is preferred, researchers at the conference said the less intensive one-week treatment is also effective.

"Even if we start treating very late in pregnancy, we can make a difference," said researcher, Dr. Joseph Saba of the United Nations AIDS program.

The study involved 1,300 woman in five urban areas of South Africa, Tanzania and Uganda

It said for the drug treatment to be successful, it must be given when labor starts and to both the mother and child for a week after birth.

Glaxo Wellcome, which combines AZT and 3TC into a single pill called Combivir, said it will lower the price for one week of treatment to less than $50 for developing countries, depending on company negotiations with each nation.

The spread of HIV during childbirth accounts for about 600,000 new cases of HIV infection around the world each year.

The Associated Press and Reuters contributed to this report.