Sydney  - We already know that smoking in pregnancy results in babies that are around 250 grams lighter than those born to mothers who did not smoke.

And we also know from the statistics that infants in homes where there are smokers are up to four times more likely to succumb to sudden infant death syndrome (SIDS) - more commonly called cot death.

Researchers in Australia believe they can explain how exposure to tobacco smoke impacts on the brains of babies.

"We think that there are certain effects on receptor systems within the brain that had led to this final mechanism of cell death," said Sydney University's Rita Machaalani.

In work just published in the journal Brain, Machaalani describes how she and her colleagues looked at the brain tissue of 67 infants who had undergone autopsies because their deaths were unexplained.

Such deaths, of which there are around 80 in Australia each year, are usually put down to SIDS.

To control for other factors, their findings were compared with an analysis of the brain tissue of 25 infants who had died suddenly but whose deaths were put down to a cause other than cot death.

They found that brain cell die-off was precipitated both by a pregnant woman smoking and by a mother exposing her infant to second-hand smoke. Cell death was in a region of the brain that regulates breathing and heart function.

"Smoking does change and affect their babies," Machaalani said, noting that around 10 per cent of Australian women smoked during pregnancy. (dpa)