Asthma may begin in the womb, say researchers who have found that children whose mothers smoke in early pregnancy are at increased risk of developing the condition.
The first 12 weeks of pregnancy − the first trimester − appears to be the critical period, the findings from over 21,000 children suggest.
Maternal smoking during these early weeks, but not during the last few months of pregnancy or the first year following birth, posed a risk.
Babies who had been exposed to smoke in this earliest stage of development went on to be two-thirds more likely to have asthma by the age of six, compared to those born to non-smoking mums.
And the more the mum smoked during early pregnancy, the greater the likelihood of the child developing a wheeze or asthma.
The findings were based on results from questionnaires sent to the parents of the children in the study.
There were 735 children whose mothers said they had smoked during pregnancy but not after giving birth.
The study authors, from the Karolinska Institute in Sweden, say some women may continue to smoke without realising that they are pregnant.
They told the American Journal of Respiratory and Critical Care Medicine that teens and young women should be encouraged to quit smoking before trying for a baby.
© Trio Media 2012