Smoking habits and birthweights in two successive births in Sweden.

A population-based longitudinal study of mothers and their first and second infant (both live and single) was conducted to investigate the change in smoking habits from one pregnancy to the next and the effect on infants' birthweights. Of the 57,732 women, 6% quit and 3% started to smoke, 61% were non-smokers and 18% were smokers during both pregnancies. Smoking information on 12% was missing. Compared with women who never smoked, the mean birthweight difference was higher among women who quit smoking and lower among women who started to smoke. Among women who quit smoking, the second birthweight was the same as among women who never smoked. The first infants of women who quit smoking were heavier than those of women who maintained smoking (3392 g vs. 3287 g). Every fourth smoker stopped smoking. Women who quit smoking differed from those who continued to smoke in terms of distribution of age and level of education. Moderate smokers quit smoking more often than heavy smokers.