We first identified 4 Reddit subreddits of which pregnant smokers might be members. We initially planned to post a link to our screener survey directly to the most promising subreddits (eg, r/BabyBumps) but were informed by moderators that this type of survey or research promotion did not comply with subreddit guidelines. We quickly discovered that Reddit has an inexpensive and flexible auction-based system for placing advertisements. We ran several advertisements on promising Reddit subreddits pertaining to smoking cessation and pregnancy, including r/BabyBumps, r/TwoXChromosomes, r/stopsmoking, and r/Parenting. We experimented with various formats, text, and images across our different ad campaigns. All advertisements provided a link to the Qualtrics screener survey. We also varied our bid price per 1000 impressions to maximize our advertising budget. Eligible respondents who completed the main survey received a US $10 e-gift cards through GiftBit, a Web-based gift card service.