Low birth weight paradox

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The low birth weight paradox is a certain observation relating birth weights and mortality of children born to smoking mothers. It is an example of Simpson's paradox.

Traditionally, babies weighing less than 2,500 grams (5.5lb) at birth have been classified as 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. The surprising observation is that low birth weight babies of smoking mothers have a lower child mortality than low birth weight babies of non-smokers. (This is not specific to smoking; poverty for example has the same effect.)

Since these findings seemed to suggest that, at least for some babies, having a smoking mother could be beneficial, some researchers have questioned the general assumption that smoking causes an increased child mortality. However, epidemiologists realize that the spurious relationship is caused by confounding by smoking.

The paradox can be simply explained by confounding: Low birth weight does not cause high child mortality, but rather the two variables (birth weight and risk of mortality) may be independently acted on by maternal smoking-- and thus, the crude association between birthweight and mortality may be partially or wholely due to correlation of each factor with smoking. Smoking of the mother lowers the birth weight, and also increases the risk of death for any given baby. As a whole, the birth weight distribution of children with smoking mothers is shifted to lower weights, moving more otherwise healthy babies into the low birth weight category, and thus lowering the child mortality of this category. 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 to 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.