Objective Pregnant women and unborn babies are at increased risk of complications from influenza, including pneumonia, yet in the UK, uptake of flu vaccination amongst this population remains <50%. Pregnant women hold beliefs about risks of flu and efficacy of vaccination that consistently predict them to decline vaccination. This study aimed to develop a theory and evidence-based intervention addressing these beliefs to promote flu vaccine uptake. Methods The intervention was developed by behavioural scientists, pregnant women, midwives, clinicians and Public Health professionals, informed by Intervention Mapping. Six predefined steps were performed in line with Intervention Mapping. Results The intervention is an animation addressing beliefs about risks of flu and efficacy of vaccination. Preliminary testing using qualitative methodology indicates the information within the animation is appropriate, and the animation is acceptable to pregnant women. Conclusions This is the first known intervention for pregnant women, aiming to increase flu vaccination through addressing risk and efficacy appraisals. It has been implemented within seasonal flu vaccination campaigns during 2018/19 and 2019/20 within one geographically and ethnically diverse area of the UK.