Commentary: Resolutions of the birthweight paradox: competing explanations and analytical insights

The analyses in Yerushalmy’s paper1 indicated that, among low birthweight infants of less than 2500 g, maternal smoking was associated with lower infant mortality. The results have been replicated in a number of studies and populations, and these seemingly paradoxical associations are now often referred to as the ‘birthweight paradox.’ As can be seen from the present set of commentaries, Yerushalmy’s paper continues to generate discussion and interest even 40 years after its publication. Several explanations have been put forward for this paradoxical association. Here we will review some of these, discuss some relations and differences between the explanations that have been proposed, and then conclude with the question of what, if anything, we can infer about the role of birthweight in governing the associations between maternal smoking and infant mortality.