Cigarette smoking and birthweight: type of cigarette smoked and a possible threshold effect.

The effects on birthweight of the number of cigarettes smoked and their tar, nicotine and carbon monoxide yields were investigated prospectively in 1309 pregnant women of whom 414 were smokers. Several approaches to modelling the effect of smoking were tried. These suggested that while both yield and quantity smoked were important, yield had the greatest effect. This led to an empirical approach whereby consistent smokers were divided into four categories according to whether they smoked a low or high quantity of cigarettes per day and whether they smoked low or high yield cigarettes. Using these four groups it emerged that women smoking a low quantity of low yield cigarettes had babies of a similar mean birthweight to those of non-smokers whereas those smoking a low quantity of high yield cigarettes had babies whose birthweight was reduced to the same degree (6% or more) as those of mothers who smoked higher quantities. This apparent threshold was estimated as 13 cigarettes/day and 15 mg/cigarette carbon monoxide. We conclude that brand smoked is at least as important as quantity and that in this population there is evidence for a threshold for tobacco smoke intake below which no discernible effect on birthweight is seen.