A treated net costs about $4 - simply too much for many African families.

This latest study drew on the results of four earlier trials in Kenya and Ghana involving more than 6,000 women.

The use of mosquito nets, treated with insecticide, it suggests, makes a big difference to the health of pregnant women and their newborn babies.

When they were used, the number of miscarriages and still-births fell by almost a third.

The number of babies born with a low weight also fell - by about a quarter.

Earlier studies have focussed on the health impact on young children - but this is thought to be one of the first to show evidence of the impact on pregnant women.

A senior author of the report, Feiko Ter Kuile, of the Liverpool School of Tropical Medicine says the scale of these studies was crucial.

"We know that malaria itself is detrimental to many women who are pregnant and results in severe maternal anaemia and reduced birth weight in children," she said.

"But we simply didn't have the definitive answers because a lot of the clinical trials themselves were not large enough to be able to answer this question."

Global programmes to reduce malaria are trying to increase the use of nets - making them more available and more affordable.