Transmission of HIV, the virus that causes AIDS, from pregnant women to their infants sometime during childbirth is a huge international problem, studies have shown.

Between 25 percent and 35 percent of babies born to untreated HIV-infected mothers become infected themselves.

"The question has always been how does the virus get from the mothers to the babies", said principal investigator Dr. Steven Meshnick, professor of epidemiology at the University of North Carolina.

Now, Meshnick and colleagues think they have discovered a major reason why that occurs. A new study they conducted with 149 pregnant HIV-infected women in the African nation of Malawi showed that tiny amounts of virus-laden blood leak from women's placentas to babies during labor.

"This work shows strongly for the first time that what we call placental microtransfusions during birth are responsible for a large part of the transmission of HIV from mother to baby", the scientist said.
"To our knowledge, there have never been any data like this before."

It appears that transmission of the virus takes place during labor, when contractions occur, rather than during passage of the baby through the birth canal.