Active maternal smoking has been shown to be associated with many complications of fetal development, and several studies consistently link respiratory tract effects, the development of allergy, and impaired lung function in school-age children with maternal smoking (Cook and Strachan 1999; Lodrup Carlsen and Carlsen 2001; National Academy of Sciences 2000). Fetal exposure to ETS in utero differs both qualitatively and quantitatively from exposure of young children to airborne ETS. It must be emphasized that these types of studies are difficult to interpret because most mothers who smoke during pregnancy also continue to smoke after a child is born (National Academy of Sciences 2000). Having said that, however, at least one study has found a stronger influence of in utero exposure than exposure encountered postnatally (Hu et al. 1997). Although maternal smoking clearly negatively impacts the respiratory health of offspring, the effects of ETS on the development of allergy and asthma are less clear, a problem possibly attributable to the difficulty of accurately isolating fetal exposure. The NAS 2000 report (National Academy of Sciences 2000) found sufficient evidence to conclude that there is an association between ETS exposure and the development of asthma in younger children.
In the limited number of studies that have been able to separate the effects of maternal smoking during pregnancy from postnatal exposure, evidence suggests that smoking during pregnancy has a stronger adverse effect on subsequent development of asthma than exposure to ETS in early childhood. Finally, it should be noted that controversy still exists in this field, for although an association between ETS and asthma is clear, at least one study suggests that mechanisms other than those associated with atopy or allergy may be involved (Lodrup, Carlsen, and Carlsen 2001).
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