Infectious diseases kill a surprisingly large number of women during pregnancy, according to a study published that suggests many maternal deaths in the developing world are preventable.

The study in the journal PLoS Medicine showed that many more women in a large Mozambique hospital died from four infectious diseases -- AIDS, malaria, bronchial pneumonia and meningitis -- than from conditions directly linked to pregnancy.

"The unexpected result was the role of the infectious disease," said Clara Menendez, an epidemiologist at the University of Barcelona, who led the study. "Over half the deaths were due to non-obstetric causes."

The diseases appear to play a similar role across sub-Saharan Africa, a region that accounts for a lion's share of the estimated 500,000 maternal deaths worldwide each year, the researchers said.

The findings add to the debate over the links between maternal deaths, HIV and malaria, and how best to allocate resources to reduce the number of women who die during pregnancy and childbirth, said Sebastian Lucas, a researcher at Kings College London, who was not part of the study.

"The conclusions were stark," Lucas wrote in a commentary in PLoS Medicine. "In contrast to received wisdom, direct maternal deaths were less frequent than indirect ones, with infectious diseases accounting for half of all deaths."

Menendez and colleagues performed autopsies on 139 of the 179 pregnant women who died at Maputo General Hospital in Mozambique's capital between October 2002 and December 2004.

Complications directly related to pregnancy and birth such as bleeding accounted for 38 percent of the deaths while infectious diseases were responsible for nearly half, the researchers said.

The findings suggest that doctors are overlooking these treatable infections, and that preventive efforts such as promoting the use of mosquito nets and condoms could help sharply reduce maternal deaths, Menendez said.

The researchers did not look at causes of maternal death elsewhere in the region but Menendez said she would expect further studies to show similar results because the diseases are so widespread.

"These deaths should not have happened," she said in a telephone interview. "Improving the access of pregnant women to all these preventable and treatable measures is critical."