Explanation
At first sight these findings seemed to suggest that, at least for some babies, having a smoking mother might be beneficial to one's health. However the paradox can be explained statistically by uncovering a lurking variable between smoking and the two key variables: birth weight and risk of mortality. Both are acted on independently when the mother of the child smokes — birth weight is lowered and the risk of mortality increases.
The birth weight distribution for children of smoking mothers is shifted to lower weights by their mothers' actions. Therefore, otherwise healthy babies (who would weigh more if it were not for the fact their mother smoked) are born underweight. They have a lower mortality rate than children who have other medical reasons why they are born underweight, regardless of the fact their mother does not smoke.
In short, smoking may be harmful in that it contributes to low birth weight, but other causes of low birth weight are generally more harmful only with regard to their weight.
Read more about this topic: Low Birth-weight Paradox
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