Aims: To introduce an 'opt out' referral pathway for smoking cessation in pregnancy and to compare different methods for identifying pregnant smokers in maternity care. Design: Pilot study that analysed routine data from maternity and smoking cessation services with biochemical validation of smoking status. Setting: Dudley and South Birmingham, England. Participants: A total of 3712 women who entered the referral pathway-1498 in Dudley and 2214 in South Birmingham. Measurements: Routine monitoring data on smoking at maternity booking, referral to smoking cessation services, number of women who set quit dates set and short-term (4-week) self-report smoking status. Comparison of self-report, carbon monoxide (CO)-validated and urinary cotinine-validated smoking status for a subsample (n=1492) of women at maternity booking. Findings: In Dudley 27% of women who entered the opt out referral pathway were identified as smokers following CO testing. Of those referred to the smoking cessation services, 19% reported stopping smoking at 4-week follow-up. In South Birmingham 17% were smokers at booking, with 5% of those referred recorded as non-smokers at 4 weeks. The number of women quitting did not increase during the study when compared with the previous year, despite higher referral rates in both areas. An optimum cut-off CO measurement of 4parts per million (p.p.m.) was identified for sensitivity and specificity. Conclusion: The introduction of an opt out referral pathway between maternity and stop smoking services resulted in more women being referred for support to quit but not higher numbers of quitters, suggesting that automatic referral may include women who are not motivated to stop and who may not engage with services. Routine carbon monoxide monitoring introduced as part of a referral pathway should involve a cut-off of 4p.p.m. to identify smoking in pregnancy.