Smoking during pregnancy can affect the baby’s immune system which may explain why asthma and respiratory problems are more common in children whose mothers smoke, Australian scientists said in a study on Thursday.

Babies of smokers are more likely to suffer from respiratory infections than children of non-smokers but until now it has not been clear why.

The scientists said it may be due to changes to biological receptors in the baby’s immune system that are responsible for recognising and fighting infections and bacteria.

“This is the first prospective study to examine the effect of smoking during pregnancy in terms of these aspects of newborn innate immune function,” said Paul Noakes of the University of Western Australia in Perth.

The researchers, who reported the findings in the European Respiratory Journal, compared 60 newborn babies whose mothers had smoked during pregnancy and 62 other infants born to non-smokers or women who had quit.

They measured the expression of several signaling compounds in the immune system linked to specific cell receptors known as TLRs in the infants.

In the babies of mothers who smoked, they discovered impaired production of two compounds, interleukin-6 and tumour necrosis factor-alpha (TNF-a).

“We were focusing on the innate, or congenital, immune system. This provides protection until the baby develops an acquired immune system, which becomes increasingly powerful through contact with new antigens,” said Susan Prescott, who also worked on the study.

The researchers said the findings show that foetal exposure to cigarette smoke is associated with changes that both weaken innate immune defenses and slow the development of the acquired immune system.