Background To promote informed decision-making and maternal pertussis vaccination (MPV) uptake, we systematically developed an interactive, web-based decision aid for pregnant users. Intervention reach (the percentage of participants in the intervention group who used the intervention), use (how much and how long those participants used the intervention), and acceptability (how positively they evaluated the intervention) are essential for it to be effective and should be reported to assess which intervention components may have been effective. Objective This is a process evaluation aiming to evaluate (1) the reach and (2) the use, and (3) the acceptability of the intervention. Methods We analyzed the reach and use of the intervention among participants in the intervention group of a randomized controlled trial (RCT) that assessed the effects of an online tailored decision aid in the form of a web app. Participants were recruited via social media and midwifery clinics and invited via email to use the intervention at 18 weeks of pregnancy. Reach was measured objectively by assessing the number of participants who visited the intervention at least once. Use of the intervention was logged and included time spent on the decision aid, the number of times clicked, pages visited, and answers given in interactive components. Data from the baseline survey (at <18 wk of pregnancy) were used to measure sociodemographics, informed decision-making, MPV uptake, and determinants of uptake. A posttest survey (20-22 weeks of pregnancy) was used to evaluate the acceptability of the decision aid. We report the findings descriptively and assess baseline differences between those who used versus those who did not use the intervention. Results Of the 586 participants in the intervention group, 463 (79%) reached the home page of the intervention. Intervention reach appeared higher among those in their first pregnancy (8.35% difference, P=.11), those recruited via their midwife rather than via social media (10.56% difference, P=.04), and those who had completed a higher educational level (7.35% difference, P=.06). On average, participants spent 4.25 (SD 4.39) minutes on the decision aid. Most participants used the decision aid once (56.2% of those who reached it, n=260) or twice (26.6%, n=123). The average number of clicks was 27.24 (SD 25.08) and varied widely. Regarding acceptability, participants evaluated the decision aid positively with an overall grade of 8.0 out of 10 (SD 1.01). In total, 38.9% (180/463) of participants who used the intervention indicated that the decision aid helped them with their MPV decision-making. Conclusions The reach of the decision aid was successful with 79%, and participants were very positive about the decision aid. The use of the intervention (eg, time spent on the intervention) leaves room for improvement and should be improved to maximize intervention effects.