The World Health Organisation refers to the weight of a baby at birth as a ‘low birth weight’ (LBW) if it is below 2.5kg, regardless of gestational age.
The mortality rate of LBW babies is significantly higher than that of non-LBW babies.
Also, the likelihood that a child born to a smoking mother has LBW is higher than that of a child born to a non-smoking mother.
Thus, one might expect the mortality rate of LBW babies of smoking mothers to be higher than that of LBW babies of non-smoking mothers.
Surprisingly, according to the observed numbers, this is not the case.
This is an example of Simpson’s paradox, a statistical phenomenon where a trend appearing in several different data groups changes when the groups are combined.
The explanation behind the LBW paradox is that other causes of LBW are generally more harmful than smoking.
Other causes include other environmental risks (such as lead exposure and other types of air pollution), drug addiction, alcohol abuse, poor nutrition, insufficient prenatal care, heart disease or hypertension, untreated coeliac disease, young ages, multiple pregnancies, previous LBW infants and pre-labour rupture of membranes.
References:
https://en.wikipedia.org/wiki/Low_birth_weight
https://en.wikipedia.org/wiki/Low_birth-weight_paradox