Background: Pregnant women are at increased risk of complications from flu, but uptake of flu vaccination is below 75% targets. Evidence suggests that changing illness risk appraisals may increase vaccination behaviour. In 2018-2019, researchers, public health specialists, and pregnant women co-designed a short animation targeting (unhelpful) beliefs underlying pregnant women's flu risk appraisals aiming to promote vaccination uptake. Aims: This study aimed to examine effectiveness of a digital intervention (animation) in increasing flu vaccination among pregnant women through changing illness risk appraisals. Method: A prospective study design was used, involving convenience sampling of unvaccinated pregnant women recruited via a Qualtrics Online Panel. Participants received small payments via the panel for survey completion. Risk appraisals and intention to vaccinate were measured at baseline and immediately after intervention presentation (follow-up one). Six months later, a further survey (follow-up two) was administered measuring vaccination behaviour. Results: Baseline and first follow-up surveys were completed by 411 participants. Watching the animation led to increased appraisals of likelihood of getting flu while pregnant and severity of flu during pregnancy, and increased intentions to accept flu vaccination during pregnancy. Of the 67 respondents who completed follow-up survey two, 38 reported having the vaccination while pregnant. Conclusions: This study provides evidence supporting the promise of the intervention. Randomized controlled trials are required to produce definitive efficacy evidence. Should such a study prove intervention effectiveness, it could be readily embedded within existing campaigns at national and local levels by public health organizations.