Multiple sclerosis (MS) in women leads to increased risk of operative delivery and reduced birth weight, which are presumably related to the neurological dysfunction in this patient group. Lifestyle factors may also contribute, and we therefore investigated smoking habits and relevant social factors in pregnant MS women. In total, 372 128 births were registered in the compulsory Medical Birth Registry of Norway from December 1, 1998 to October 6, 2005, and of them 250 by MS mothers. The MS births were compared with all the non-MS births. Smoking during pregnancy was not increased in the MS group compared with the non-MS references. From 1998 to 2005 the MS group had a larger reduction in smoking rate during pregnancy than the reference group. The differences in pregnancy and birth outcome between smokers and non-smokers were similar in the MS and the reference group. Those in the smoking MS group had no increase in birth complications, operative interventions or negative birth outcome compared with those in the smoking reference group. Smoking during pregnancy did not explain the birth weight reduction found for newborns of MS mothers.