The low birth weight paradox
is an apparently paradoxical
observation relating to the birth weights
and mortality of children born to tobacco smoking
mothers. Low birth weight
children born to smoking mothers have a lower infant mortality
rate than the low birth weight children of non-smokers. The same is true of children born to poor
parents, and of children born at high altitude.
Traditionally, babies weighing less than a certain amount (which varies between countries) have been classified as having low birth weight. In a given population, low birth weight babies have a significantly higher mortality rate than others. Populations with a higher rate of low birth weights typically also have higher rates of child mortality than other populations. The children of smoking mothers are more likely to be of low birth weight, and also have a higher child mortality. So it is a surprising real-world observation that low birth weight babies of smoking mothers have a lower child mortality than low birth weight babies of non-smokers.
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 be fatter 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, a smoking mother is harmful in that it contributes to low birth weight, but other causes of low birth weight generally are far more harmful.
If one corrects and adjusts for the confounding
by smoking, via stratification or multivariable regression modelling to statistical control for smoking, then one finds that the association between birthweight and mortality may be attenuated towards the null. Nevertheless, most epidemiologic studies of birthweight and mortality have controlled for maternal smoking, and the adjusted results, although attenuated after adjusting for smoking, still indicated a significant association.
Additional support for the hypothesis that birth weight and mortality can be acted on independently came from the analysis of birth data from Colorado: compared with the birth weight distribution in the US as a whole, the distribution curve in Colorado is also shifted to lower weights. The overall child mortality of Colorado children is the same as that for US children however, and if one corrects for the lower weights as above, one finds that babies of a given (corrected) weight are just as likely to die, whether they are from Colorado or not. The likely explanation here is that the higher altitude of Colorado affects birth weight, but not mortality.