Introduction: The Medical Birth Registry of Norway provides national coverage of all births in Norway. In this study trends of smoking at the beginning of pregnancy and the variation of these trends across different sociodemographic groups are analyzed. This knowledge is important for planning interventions both in the general population and specifically among pregnant women. Methods: All births registered in Medical Birth Registry of Norway 1999-2014 with information on mothers' smoking status were included in the overall analyses of the smoking trends (806 298). Records from 210 268 births in two time periods 1999-2000 and 2013-2014 were selected, and for the multivariate analyses we used general linear models to provide adjusted risk ratios. Results: The prevalence of maternal smoking at the beginning of pregnancy decreased from 25% to 8% over the 15-year period. Mothers with low, medium, and high education, respectively, had a 46% (RR 0.54, CI 0.52-0.55), 62% (RR 0.38, CI 0.37-0.40), and 80% (RR 0.20, CI 0.19-0.22) reduction in maternal smoking between the two time periods. Similarly, the decline in smoking was greater for mothers who were married or living together (64%; RR 0.36, CI 0.35-0.37) than for single mothers (39%; RR 0.61, CI 0.58-0.64). Immigrants had a lower smoking prevalence than Norwegians in the entire period 1999-2014. Conclusion: The prevalence of smoking during pregnancy in Norway decreased in all population groups, but the relative differences between the educational groups increased from 1999-2000 to 2013-2014. This is a public health concern and a strong contributor to health disparities. Implications: Women with low and medium education level and single mothers had the highest smoking prevalence and the lowest decline over the two time periods. These groups constitute 45% of our study population, the relatively high maternal smoking in these groups continues to be of public health concern for maternal and child health. © The Author 2017.

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educational status immigrant Norway demography married woman human epidemiology Odds Ratio Pregnant Women statistics and numerical data controlled study social aspect priority journal Norwegian (citizen) Logistic Models social support Humans psychology smoking Infant, Newborn maternal smoking female Socioeconomic Factors newborn socioeconomics pregnancy Mothers Infant, Low Birth Weight prevalence Article low birth weight pregnancy outcome mother adult single parent pregnant woman statistical model adverse effects