Environmental Tobacco Smoke and Birthweight of Offspring: A Critical Review and Analysis of the Epidemiological Literature

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Abstract
Forty papers on the association between environmental tobacco smoke (ETS) exposure of the mother and low birthweight of offspring were examined from the point of view of experimental design, criteria for ETS exposure, sample size, methods of statistical analysis, individual confounders considered, spe cific endpoints, and outcome. An approximately equal number of studies employed either the prospective or retrospective design, with very few using the case-control design. The vast majority of studies used a surrogate for ETS exposure (mainly paternal smoking), without verification of such exposure with a biochemical marker. The sample sizes of the studies ranged from under 100 subjects to almost 25,000 subjects. Most studies employed regression analysis. A considerable variation from study to study was noted in the treat ment of potential confounding variables and most studies considered relative ly few of these. Most of the individual confounding variables were considered in a relatively small proportion (one third or less) of the studies. The majority (almost two thirds) of the studies failed to demonstrate a statistically signifi cant association between ETS exposure of the pregnant woman and low birth weight. This general lack of consistency of association could reflect absence of an adverse effect of ETS or methodologic difficulties such as misclassification error, type II error, recall bias, or inadequate adjustment for potential con founders.