Birthweight and cigarette smoking
Recent research has shown an association between smoking in pregnancy and low birthweight. Many authors have concluded that the relationship is causal but some have argued that it is the smoker rather than the smoke which is responsible. This thesis examines the relationship between the smoking habit in pregnancy and birthweight corrected for gestational age using data from the St. George's Hospital Birthweight Study. Adjustment is made for confounding factors so that the effect of smoking can be estimated. The statistical problem of adjusting birthweight for gestational age when very early births are included is discussed and a solution presented in the form of a birthweight ratio. The relationship is examined between birthweight ratio and many socioeconomic and psychological factors and shows that few are associated with reduced birthweight. Those associations which are observed can be explained by smoking. Alcohol and caffeine are only related to birthweight in smokers. When the smoking habit is analysed in terms of quantity and constituents, a threshold is observed whereby women smoking a low number of low yield cigarettes have mean birthweight similar to non-smokers. For women smoking higher numbers of cigarettes but a low yield brand mean birthweight is reduced by the same amount (6% or more) as women smoking high yield brands. The effect on birthweight of alcohol and caffeine in smokers only is adjusted for smoking by using this threshold. This shows that smoking, alcohol and caffeine are all associated with reduced birthweight. For alcohol and caffeine consumption this relationship is strongest in early pregnancy and weakest near delivery. The association between smoking and birthweight is not explained by any of the wide range of confounding factors examined. This provides evidence that the relationship is a causal one.