Of 4341 pregnant women, 3106 were non-smokers and 1235 were smokers at the start of pregnancy. Eighty-five had stopped smoking before 6 weeks gestation, 119 between 6 and 16 weeks, and 56 stopped after 16 weeks. A further 51 stopped temporarily and 924 smoked throughout pregnancy. The mean birthweight of the groups differed. There were also social and biological differences such as might partly explain birthweight variations so comparisons were repeated after allowing for these factors. Standardized mean birthweights were greater for all groups who stopped than for persistent smokers. Those who stopped before 6 weeks and between 6 and 16 weeks gestation had infants 217 and 213 g, respectively, heavier than the persistent smokers and similar to the non-smokers. Babies born to those who stopped after 16 weeks, or temporarily, were intermediate in weight. Appropriate advice is that stopping any time before 16 weeks is best, but stopping after this is still beneficial.
PIP: This paper examines the effect on birth weight of stopping smoking at different stages of pregnancy: before 6 weeks, between 6 and 16 weeks, and after 16 weeks. The study was done on 1235 smokers delivered at a West Midlands maternity hospital. They were divided into 5 groups: 85 women who stopped smoking during the 1st 6 weeks, 119 women who stopped between 6 and 16 weeks, 56 women who stopped after 16 weeks, 51 women who stopped during the 1st 3 months but resumed smoking before delivery, and 924 women who smoked throughout pregnancy. The average number of cigarettes smoked per day was between 13.1 and 13.8 for the 1st 3 groups, 15.5 for the group that stopped temporarily, and 18.6 for the group that smoked throughout pregnancy. In age, height, and social class the 1st 2 groups resembled the nonsmokers, and the 3rd group resembled the persistent smokers. The 1st 2 groups, however, included more nulliparas than did the nonsmokers; the 3rd group included more nulliparas than did the persistent smokers; and the temporary stoppers were intermediate between the 3rd group and the persistent smokers. All the groups who stopped smoking had bigger babies than did the persistent smokers, but the babies of the temporary stoppers were not significantly bigger. The babies of those who stopped smoking before 6 weeks or between 6 and 16 weeks were 217 gm and 213 gm respectively heavier than the babies of the persistent smokers. The babies of those who stopped after 16 weeks were 120 gm heavier than those of the persistent smokers but 100 gm lighter than those of the early stoppers. The babies of the temporary stoppers were similar to those of the late stoppers. Stopping smoking at any time up to 30 weeks results in increased birthweight, but the greatest effects result from stopping before 16 weeks.