Smoking during pregnancy linked to asthma in offspring

Robin Wulffson MD's picture
pregnancy, smoking, asthma, secondhand smoke
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A number of studies have reported that children of mothers who smoke during pregnancy are more likely to develop asthma than children of non-smokers. However, it was unclear from these studies whether this was primarily due to exposure to secondhand smoke after birth. A new, large study has reported that children of mothers who smoke during pregnancy have an increased risk of developing asthma even if they are not exposed to secondhand smoke. The findings were published online on August 17 in the American Journal of Respiratory and Critical Care Medicine.

Anna Bergstrom and her colleagues at the Karolinska conducted a study that contained a large number of children who were exposed to smoke in the uterus but were not exposed to secondhand smoke after birth. The study group was comprised of 21,600 European children, including 735 children whose mothers claimed that they had smoked during pregnancy but not after giving birth. It found that those 735 were two-thirds more likely to have asthma by age six, compared to children of mothers who did not smoke during pregnancy. Even smoking just during the first trimester (first three months of pregnancy) was associated with a higher asthma risk. Of the mothers who claimed that they smoked only during pregnancy, most quit during the second or third trimester. However, first-trimester smoking alone was linked to a two-fold increase risk of the child developing asthma.

By the time the children in the study group were between the ages of four and six, almost 7% of them had been diagnosed with asthma; however, the risk was higher among children whose mothers smoked during pregnancy. These children were 65% more likely to develop asthma than children of non-smokers when factors such as birth weight and both parents’ own history of asthma were taken into account.

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The researchers note that their study suggested a correlation between smoking mothers and asthmatic children; however, it cannot prove cause-and-effect. Despite the inability to prove cause-and-effect, they note that it is likely that prenatal smoking raises a child’s future asthma risk. It is unclear how smoking can increase asthma risk; however, it is theorized that chemicals in tobacco smoke may affect early lung development.

Take home message:
I find it surprising that women who smoked during pregnancy would stop smoking after pregnancy. In my obstetrical practice, most women who smoked quit when they found out they were pregnant. Maternal instincts prompted them to adopt a healthy lifestyle. However, after the pregnancy, some resumed practices they deemed harmful during pregnancy. In addition, even if a smoker kicks the habit after pregnancy, other household members may smoke; thus, exposing the child to secondhand smoke. All things considered, however, it is likely that smoking during pregnancy increases the risk of childhood asthma. Chemicals in tobacco smoke enter the mother’s blood stream and are passed into the fetal blood stream via the placenta. In addition, to the chemical exposure, nicotine causes vasoconstriction (narrowing of smaller arteries). This results in decreased blood flow—and oxygen—to the fetus. Thermography, which measure temperature levels have been taken while a pregnant woman smokes. With the first puff, a demonstrable cooling, indicative of decreased placental blood flow occurs. In essence, smoking during pregnancy is child abuse.

Reference: American Journal of Respiratory and Critical Care Medicine

See also:
Smoking during pregnancy causes respiratory problems in offspring
Screening for smoking may reveal substance abuse during pregnancy
Preventing substance abuse in pregnancy: a successful program

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