If you're pregnant and smoking, you're smoking for two, and your baby may develop the asthma to prove it, suggests new research.
And, the child's asthma will get worse if it's exposed to more tobacco smoke after birth, say the researchers.
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It's no secret that tobacco smoke is bad for children diagnosed with asthma; it triggers more symptoms and requires more drugs and trips to the emergency room.
But until now, it's been hard to separate the two phases of smoke exposure: before birth and when breathing second-hand smoke after birth.
Lead investigator Dr Frank Gilliland studied how tobacco smoke exposure affected children early in life with data from the 10-year Children's Health Study, a long-term follow-up of 5 762 children from 12 Southern California communities.
Gilliland and his team analysed questionnaires filled out by the children's parents about each child's lifetime tobacco exposure, including whether the mother smoked during pregnancy, as well as information about whether the child wheezed or had been diagnosed with asthma.
They found 18.8 percent of the children were exposed to smoke while in the womb, and 39.5 had some lifetime exposure to environmental tobacco smoke (ETS). Of the children exposed in the womb, 17.8 percent were told by doctors they had asthma, while 38.3 percent of those with later exposure to smoke had a history of wheezing.
"What we found was that exposure during the in-utero period was very important for subsequent development of asthma, as well as the severity of asthma later on," says Gilliland. "Environmental tobacco smoke added to that. … If you were exposed both as a foetus in the womb and later in life, then you got a double whammy."
The researchers don't completely understand what effects the mother's smoking has on the developing lungs of a foetus. "We do know that it causes smaller lungs; it causes the small airways to be abnormal," says Gilliland.
"One possible explanation is the airways [in general] are much smaller. They also appear to be much more responsive to insults, and that makes [the children exposed in utero] more likely to have asthma."
Dr Jonathan Samet, chairman of the Department of Epidemiology at Johns Hopkins University, says other studies have shown that ETS can affect lung growth, increase risk for respiratory infection, and make children more likely to cough or wheeze and possibly increase the risk for asthma.
"This is another piece of evidence consistent with the idea that mom's smoking during pregnancy increases the risk for asthma, and it's clearly a lot of evidence that smoke exposure anytime increases risk for symptomatic wheezing," says Samet.
"One of the difficult questions has been trying to sort out how much of that represents an effect of smoke exposure after birth versus some carry-over effects from exposure during pregnancy. This study attempts to get at that," he says.
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