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.
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.
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.
Birth weight, math and reading achievement growth: a multilevel between-sibling, between-families approach.(Report)
Mar 01, 2009; The adverse impact of infant health on development is one of the many mechanisms through which childhood disadvantage is thought...