Formative pilot testing of RCTs in the target group is an important first step in developing intervention approaches most likely to be feasible, appealing to, and effective in the target group [34]. Small-scale randomised controlled trials (RCTs) are suitable for feasibility studies of high internal validity when they closely approximate the clinical or community context of a larger scale RCT [34]. Thus, the aim of this study is to test the feasibility of an mHealth intervention to promote healthy nutrition, physical activity and weight gain in pregnant women who are overweight or obese prior to pregnancy.