OBJECTIVES: To report the methods used to assemble a contemporary pregnancy cohort for investigating influences on smoking behaviour before, during and after pregnancy and to report characteristics of women recruited. DESIGN: Longitudinal cohort survey. SETTING: Two maternity hospitals, Nottingham, England. PARTICIPANTS: 3265 women who attended antenatal ultrasound scan clinics were offered cohort enrolment; those who were 8-26 weeks pregnant and were currently smoking or had recently stopped smoking were eligible. Cohort enrollment took place between August 2011 and August 2012. PRIMARY AND SECONDARY OUTCOME MEASURES: Prevalence of smoking at cohort entry and at two follow-up time points (34-36 weeks gestation and 3 months postnatally); response rate, participants' sociodemographic characteristics. RESULTS: 1101 (33.7%, 95% CI 32.1% to 35.4%) women were eligible for inclusion in the cohort, and of these 850 (77.2%, 95% CI 74.6% to 79.6%) were recruited. Within the cohort, 57.4% (N=488, 95% CI 54.1% to 60.7%) reported to be current smokers. Current smokers were significantly younger than ex-smokers (p<0.05), more likely to have no formal qualifications and to not be in current paid employment compared to recent ex-smokers (p<0.001). CONCLUSIONS: This contemporary cohort, which seeks very detailed information on smoking in pregnancy and its determinants, includes women with comparable sociodemographic characteristics to those in other UK cross-sectional studies and cohorts. This suggests that future analyses using this cohort and aimed at understanding smoking behaviour in pregnancy may produce findings that are broadly generalisable.