Methods	Randomised controlled trial in Greater Burlington, Vermont, USA. 2001-2003
Participants	Participants were recruited from 1 of 4 large obstetric practices. Inclusion criteria: self-reported smoking (even a puff in the last 7 days), gestational age less than 20 weeks, living within study clinic county and not planning to move until at least 6 months postpartum, English speaking, not incarcerated and not previously participating in the study or living with anyone who has previously participated in the study 182 women were eligible for the study, and 82 (45%) agreed to participate. 5 women withdrew from the study due to fetal demise or termination of pregnancy and were not included in the final analysis (I = 3, C = 2) There was no significant difference in baseline characteristics of the groups, including pre-pregnancy cigarettes per day (I =18.7, C =18.4), health insurance (I =19, C =13), and timing of recruitment (I = 8.9, C = 9.5)
Interventions	Intervention (contingent voucher): participants chose a quit date, and reported daily to the clinic for CO monitoring for 5 days, then urine cotinine monitoring twice weekly for 7 weeks, weekly for 4 weeks, and then every 2 weeks for the remainder of the pregnancy. Vouchers were given dependent on biochemical validation, beginning at US$6.25 and escalated by US$1.25 to a maximum of US$45.00. Positive test results reset voucher back to original value, but two consecutive negative tests restored value to pre-reset value Control (non-contingent voucher): Participants received voucher independent of smoking status. US$15.00 per antenatal visit and US$20.00 per postpartum visit, to result in a comparable average earnings to the contingent group Both groups received routine advice from the clinic. It is unclear who delivered the intervention. The theoretical basis on the intervention is rewards and feedback The intensity rating: I = 4, C = 4.
Outcomes	Smoking cessation at 28 weeks’ gestation, 12 weeks and 24 weeks’ postpartum Reduction in mean cotinine. Mean birthweight, gestational age, fetal growth measures (US), and proportion of NICU admissions and low birthweight babies
Notes	Sample size justification. Process evaluation not reported. Some discussion of cost implications.
Risk of bias
Item	Authors’ judgement	Description
Adequate sequence generation?	Unclear	Described as “randomisation stratified to clinics”. Details of randomisation not described
Allocation concealment?	Unclear	No information.
Blinding? Women and clinical staff	No	Participants and providers not blinded as receiving incentives for participation
Incomplete outcome data addressed? All outcomes	Yes	Small loss to follow up due to pregnancy termination or fetal death. Available case analysis
Free of selective reporting?	Yes	Detailed birth outcomes reported.
Free of detection bias?	Yes	Exhaled CO for 5 days (< 6 ppm) and then urine cotinine (< 80 ng/ml)