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Research showing children aged under two are four to five times more likely to get a brain tumour if their mother smoked when pregnant needs further investigation, the researchers say.

The study, led by Telethon Institute for Child Health Research Professor Elizabeth Milne, also found the risk for children aged under five almost doubled.

Co-author Professor Bruce Armstrong, of the Sydney School of Public Health, said the finding on infants under two was based on a small sample and suggested more research was needed into the association between maternal smoking and childhood brain tumours.

"It is only when we look at the relationship between risk of brain tumours, mothers smoking and the age of the child that we see that under five years of age there is evidence of an increased risk of brain tumour for children whose mothers smoke during pregnancy," Professor Armstrong said.

"That seems to be stronger as you go down to the younger ages.

"This suggests there is an increase in risk of brain tumours in younger children when their mother smokes during pregnancy.

"But that's a result that would have to be confirmed in additional investigations."

The study, published recently in the International Journal of Cancer, is the first of its kind in Australia.

It points to studies on animals that say "the developing brain is much more likely to develop tumours as a result of neurocarcinogens in utero than in later life and this may also be the case for humans".

The authors said the study provided little evidence that parental smoking at preconception or in pregnancy was associated with an increased risk of childhood brain tumours overall, except for the infants of smoking mothers.

"Childhood brain tumours are the leading cause of cancer death in children, yet their etiology remains largely unknown," they said. "We recommend that future studies investigate the associations between maternal smoking and risk of CBT by the child's age of diagnosis."

Professor Armstrong said previous studies into the area had inconsistent results.

"There isn't among the studies done so far a clear consensus that there is this increase in risk," he said.

"But smoking in pregnancy is definitely bad for the baby.

"This gives mothers another reason why giving up smoking if they are thinking of having a baby, or not smoking at all, is a really good choice."