Smoking has been shown to affect glucose homeostasis and increase the risk for type 2 diabetes mellitus. Further, gestational diabetes mellitus (GDM) and smoking are known to influence offspring birthweight. The effect of smoking on glucose homeostasis in pregnancy is less studied and the findings are inconsistent. The aim of this study was to evaluate the effect of smoking on risk for GDM and to evaluate the impact of smoking and GDM on offspring birthweight. This is an observational cohort study encompassing 4111 Finnish primiparous women from the city of Vantaa, Finland, who delivered a singleton child between 2009 and 2015. Data were obtained from Finnish national registers. Study participants had complete oral glucose tolerance test results and were divided into three groups according to smoking status: non-smokers (I), smokers who quit during first trimester (II), and smokers who continued after first trimester (III). Prevalence of GDM was 19.8%, 24.3%, and 26.6% in non-smokers, those who quit, and those who continued after the first trimester, respectively (P = .004 for differences between groups). The odds ratio for GDM in smokers who continued after the first trimester compared with non-smokers was 1.65 (95% CI 1.09-2.57) after adjustments for age, prepregnancy body mass index, education, and cohabitation. In women without GDM, offspring birthweight was lowest in those who continued smoking after the first trimester (P = .010 for differences between groups). In women with GDM, smoking status did not influence offspring birthweight. Smoking during pregnancy is associated with an increased risk for GDM. Offspring birthweight is lowest in women who continue smoking after the first trimester. If pregnancy is complicated by GDM, offspring birthweight is not influenced by smoking. © 2020 The Authors. Acta Obstetricia et Gynecologica Scandinavica published by John Wiley & Sons Ltd on behalf of Nordic Federation of Societies of Obstetrics and Gynecology (NFOG).