Background: Reductions in maternal smoking can prevent pregnancy complications and adverse effects to foetus. Our objective was to study how the prevalence of maternal smoking differs between Nordic countries, and to identify target groups for smoking-cessation interventions. Methods: Information on maternal smoking and background factors was requested from the Nordic countries (the Danish National Board of Health, the Finnish National Institute for Health and Welfare, the Public Health Institute in Iceland, the Norwegian Institute of Public Health and the Swedish National Board of Health and Welfare). Data on maternal smoking were received from 1991 to 2010 in Denmark, 1987 to 2010 in Finland, 1999 to 2009 in Norway and 1983 to 2008 in Sweden. Trends in smoking were studied by using test for relative proportion. Results: The prevalence of maternal smoking in early pregnancy has declined in the countries during the past 20 years (Denmark: from 30.6 to 12.5%; Norway: 20.6 to 16.5% and Sweden: 31.4 to 6.9%), except in Finland (a steady prevalence at 15%). The highest rates of smoking in early pregnancy were among teenagers (24% in Sweden and 49% in Finland and Norway). Single women were 2–3 times more likely to smoke than married women. The women in the lowest socioeconomic group were 6–7 times more likely to smoke than women in the highest group in Finland and Norway. Conclusion: Maternal smoking and its trends differed between the Nordic countries. The highest smoking rates during pregnancy were observed among teenagers, single women and women with a low socioeconomic position.