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.