Background: Many pregnant women smoke despite the extensive data available on the detrimental perinatal outcomes. The aim of this study was to estimate the prevalence of smoking during pregnancy and to identify the factors associated with smoking among pregnant women in Northern Greece.Methods: A sample of pregnant women (mean gestational age: 12.6 weeks) participated in a cross-sectional study assessing the prevalence of smoking. Univariate and multivariate logistic regression analyses were used to examine the factors associated with their smoking behavior.Results: A total of 3688 pregnant women (mean age: 31.1 ± 5.0 years) completed the survey. On the basis of their answers, 36.1% (n = 1330) were smokers before pregnancy and 13.2% (n = 487) were still smoking at the end of the first trimester. Eight hundred and forty-three women (63.4%) gave up smoking, apparently motivated by their pregnancy. A multivariate analysis showed an independent positive association of nulliparity, Greek ethnicity, age >35 years and a BMI > 30 with smoking before pregnancy. Smoking during pregnancy was more common among naturally conceived pregnancies and women with a BMI > 30.Cessation rates were lower for naturally conceived pregnancies, immigrants and multiparous women.Conclusion: About one third of women who smoked before pregnancy continued to smoke during pregnancy. Naturally conceiving, multiparous and immigrant women are less likely to quit smoking when pregnant. Tailored smoking cessation interventions should target women early in their pregnancy.