The study will be completed digitally, among participants who have been referred to the study through partner organizations in 24 counties throughout California.

Youth must meet the following criteria to be eligible for the study: (1) age 16 to 19 years at time of study enrollment; (2) currently in foster care or formerly in foster care (and were in foster care on or after their 13th birthday); (3) currently residing in California; (4) proficient in English; and (5) have access to a tablet, laptop, desktop computer, or phone that is Wi-Fi- or cellular-enabled.

They also had to be free of severe mental health issues and have the cognitive ability to meaningfully engage with the web platform as determined by the recruitment ambassadors.

Participants randomized to the active intervention condition will receive access to Next4You, a fully mobile intervention designed to reduce rates of unintended pregnancy and STIs among young people currently or recently in the California foster care system. The platform was designed in partnership with foster care youth and with input from community-based organizations serving this population. The responsive, web-based platform can be accessed by young people at any time from an internet-enabled mobile device.

The intervention features 6 content modules (Table 1): Sexual Wellness, Contraception, Relationships, Respecting Self and Partners, Communication, and Building Wealth and Wellness (eg, financial literacy, educational rights, and mental health), with a total instructional time of approximately 3 to 4 hours. Each module includes 5‐12 microlessons, designed as brief, informal snippets of content that are 5 minutes or less; focused on a single learning concept; and are delivered through diverse approaches such as games, videos, quizzes, and others [29]. One advantage of this method includes the repeatability of the content in time, place, and format preferred by most young people [30]. In addition to microlearning, the site includes gamification features, a youth-designed retention strategy to increase motivation to engage youth in the content.

The Next4You platform includes a total of 50 microlessons, and participants earn points for each completed lesson. To motivate early engagement with the website, participants receive a US $25 e-gift card upon completing 5 lessons. Subsequent points are banked, and the remaining incentive amount is distributed at the end of the 4 weeks, based on the total amount of points earned. Participants may earn up to US $100 in e-gift cards for completing all 50 microlessons. In addition to lesson completion, participants can earn up to US $25 in bonus incentives through all 4 achievement badges: Start Up, On Fire, My Faves, and Consistency. In total, participants may earn a maximum of US $125 in e-gift cards over the 4-week program period.

The online platform will collect paradata in the form of timestamps and counts of user sessions for all actions performed by each user, thereby allowing for calculation of the time spent engaging with the intervention (including total time spent on each microlesson, modules, and engaging with various activities and resources), frequency of logins, and the amount of completed content. Users will be automatically logged out after approximately 10 minutes of inactivity to protect privacy and reduce the likelihood of inflated duration data. The evaluation will use the duration data to conduct exploratory analyses examining differing levels of exposure and the relation to study outcomes.

The development of the Next4You intervention draws upon other pertinent knowledge bases to center youth voice through a youth participatory process and draws on the latest research in positive youth development and microlearning to strengthen the intervention’s content and strategies (see the screenshots in Multimedia Appendix 1). Positive youth development is a strength-based approach used to promote adolescents’ prosocial competencies and build skills related to their positive health and well-being [31]. The adolescent development literature guided Next4You’s relationship development content and helped ensure it is age appropriate [23]. The intervention also draws on dual process theories to address the social-emotional and cognitive influences on sexual decision-making [32]. Finally, the intervention draws from social cognitive theory both in terms of key constructs in skill acquisition, such as building self-efficacy or confidence in one’s ability to perform a skill, and in shaping the process of learning, such as through using observational learning or modeling during instruction [33].

Engaging youth with lived experience in the foster care system as subject matter experts was an essential first step in developing the Next4You platform and its content. This approach provided a unique opportunity to deliver accurate, inclusive, culturally competent, and on-demand information, resources, and tools tailored to the needs of transition-age youth. To refine and enhance the platform before piloting, the team used youth participatory methods to engage these subject matter experts. These methods incorporate positive youth development principles, emphasizing the importance of engaging young people as partners by respecting their knowledge, insights, expertise, and leadership [34]. Recognizing that youth in foster care often experience trauma, we used trauma-informed and healing-centered principles when engaging co-designers, including safety, equity, empowerment, and resilience. This ensured that the co-design process was inclusive, culturally relevant, and supportive, even when difficult topics arose.

The co-design process involved 9 participants, ages 19‐22 years, from California, who engaged individually and as part of a group over 7 weeks through weekly 90-minute meetings. The facilitation approach was intentionally structured yet flexible. The sequence included 2 sessions on discovery (eg, community building), 2 on defining (research and product design and function), and 3 on designing content for intervention modules, concluding with a design and testing session. Slide decks structured each meeting, but sessions were highly interactive, incorporating icebreakers, small-group discussions, and digital tools (eg, Mentimeter or Kahoot). Participants were invited to be active cocreators rather than passive reviewers; facilitators used semistructured prompts such as “What keeps users coming back?” or “What does wellness mean to you?” to elicit ideas that shaped both content and user experience. Through this process, participants influenced the platform’s esthetics, navigation, and functionality, including customizable color schemes, authentic imagery, gamified features, and microlesson delivery. They identified priority content areas and co-developed stories, scenarios, and audio narration reflecting real experiences. The team used mockups and mood boards to align design decisions with participant input and maintained engagement between sessions through asynchronous channels (eg, GroupMe). Following co-design, pilot testing of 18 lessons yielded highly positive feedback. Participants recommended adding more interactive activities, videos, and visual animations, as well as expanding life-skills content such as improving communication with adults in positions of authority including teachers and supervisors. These refinements strengthened both usability and relevance for youth transitioning from foster care.

By centering youth as co-designers, the Next4You project produced a platform that authentically reflects their experiences and priorities, demonstrating the power of participatory design to advance youth-driven innovation in adolescent health.

Youth randomized to the comparison condition will have access to a mobile-responsive website featuring digital resources focused on nutrition, sleep, stress and anxiety, and exercise. Consistent with the intervention group, participants in the comparison group will have unlimited access to the general health digital resources for the initial 4-week period. Throughout the evaluation, youth assigned to the control condition will also continue to receive the standard resources and services typically available to them through their involvement in the foster care system. The comparison condition was selected to establish an effective treatment contrast [35] by isolating the difference between the treatment received by youth with access to the Next4You platform and the treatment received by youth who instead received access to digital resources focusing on nutrition, sleep, stress and anxiety, and exercise. After the initial 4-week intervention phase, comparison group participants will retain access to the comparison website. Upon completion of the 9-month evaluation period, these participants will additionally receive access to the Next4You intervention website for the remainder of the study, concluding on September 29, 2026.

The evaluation of Next4You will rely on a confidential youth self-report survey assessing knowledge, attitudes, beliefs, and behaviors related to sexual health through web-based surveys administered via the Qualtrics platform. Young people receive a unique link via email to the survey immediately after affirming their assent to study participation. The study team will follow up with youth who do not complete the baseline survey by sending up to 3 texts and 2 additional emails over the course of approximately 3 weeks, each with the unique link to their baseline survey, and an offer to answer questions or support in completing the survey. The study team will then engage the recruitment ambassador for assistance in reaching out to the youth and encouraging them to complete the survey.

Participants will be randomized after the baseline survey and are automatically re-directed at the end of the survey to access their assigned intervention condition. The baseline survey is estimated to take approximately 25 minutes to complete. Participants receive a gift card worth US $40 for filling out the baseline survey.

One month prior to sending each follow-up survey, youths are asked to provide the study team with their latest contact information. Each youth is contacted simultaneously at their last known phone number and email address—one of which has usually not changed in the 3- and 6-month periods since their last follow-up survey. This outreach serves to both remind youth that a follow-up survey will be sent to them in the next few weeks and maintain the most recent, preferred contact information to reach the youth when their next survey is sent. Doing so 1 month in advance gives youth time to reply or receive up to 3 reminders if needed. This confirmation of contact information request is associated with a US $10 e-gift card when the youth replies to the study team with their current contact information.

Both follow-up surveys are administered online using procedures like those used at baseline. The study team will use a multimethod outreach process to contact youth who do not respond to the surveys. In the first phase, the study team will send up to 3 simultaneous texts and emails to encourage nonrespondents to complete the survey. During the second phase, youth who have not responded will receive up to 2 calls (with voicemails) from the study team during the following week. If youth have not completed the survey during the second phase, the study team will wait 1 week before implementing the third phase of outreach, which involves engaging with the study recruitment ambassadors for assistance in reaching out to the youth and encouraging them to complete the survey. At all outreach attempts by the study team, participants are provided with the study team’s email and phone for any questions or need for assistance in completing the survey.

Given the intervention is fully mobile, our implementation evaluation data collection focuses on four areas related to fidelity that have been found to influence dose and treatment effects in mobile interventions [37]: (1) amount (count of lessons completed), (2) frequency (counts of user sessions and patterns of use), (3) duration (amount of time spent in each module and lesson), and (4) depth (total content and engagement as a percentage of all available content). These data will be collected from the web platform directly. We will also conduct selected postintervention interviews to gather qualitative data on user experiences during the study.

The study team will implement a comprehensive participant tracking and retention strategy designed to maximize response rates and minimize attrition across all follow-up periods. This approach includes allocating sufficient resources for incentives, dedicating staff time to personalize follow-up, and conducting multiple outreach attempts to nonrespondents. Consistent with intent-to-treat (ITT) principles, all youth enrolled in the evaluation will be tracked and will receive retention-focused outreach, regardless of level of engagement or participation.

Participants will receive e-gift card incentives for completing evaluation surveys, verifying or updating contact information, and completing intervention activities. In total, youth may earn up to US $315 for completing all activities. Incentive amounts include US $40 for the baseline survey, US $60 each for the 3- and 9-month follow-up surveys, US $10 per response (up to 2 times) for confirming or updating contact information, and up to US $125 for completing intervention activities. Incentive values were informed by a consultant with lived and professional experience in the foster care system to ensure that compensation reflects and honors participants’ time and expertise.

Retention in the intervention group platform will include gamification features such as badges and a point-based reward system that allows for points to be converted into electronic gift cards. Additionally, push factor text and email notifications will be used to promote engagement. For example, if there has been no activity for at least 3 days, an automatic notification will go out to participants inviting them to log onto the platform and continue earning points through the completion of lessons. Youth assigned to the comparison group platform will receive equivalent total incentives for completing a minimum of 12 online pamphlets [38-49] on promoting healthy eating and nutrition, sleep, exercise, and reducing stress and anxiety. The total amount of incentives that youth may receive is the same for both study groups.

The primary outcome of this study is the impact of the 4-week Next4You intervention on contraceptive use among youth in foster care. Specifically, this study will examine the prevalence of participants who report either abstaining from sex or using c condoms during vaginal or anal sex in the past 3 months, compared with participants provided access to a mobile-responsive website containing only general wellness resources (sleep, nutrition, stress and anxiety, and exercise). This outcome will be assessed using data from the 3-month postintervention follow-up survey.

The secondary outcomes will examine the impact of the 4-week Next4You intervention, compared with a mobile-responsive website on wellness topics, on a range of sexual and reproductive health and well-being outcomes among youth in foster care. Specifically, analyses will assess effects on condom use behaviors, condomless sex, and condom errors; self-efficacy related to sexual consent and communication; attitudes toward condom and contraceptive use; power balance and experiences within sexual relationships; and knowledge of health care rights, educational rights, service locations, and financial literacy. Outcomes will be assessed at 3- and 9-month postintervention follow-ups.

Multimedia Appendix 2 [50-57] highlights our primary and secondary measures, with sample survey items.

Our analysis plan follows a series of analytic steps, including data cleaning, analysis of baseline equivalence and attrition, handling of missing data, descriptive analysis, impact analyses of confirmatory research questions, and exploratory analyses. Throughout the analysis phase, we will use a significance level of 0.05 and employ an ITT framework, whereby young people who were randomly assigned to a condition are considered to be in that condition regardless of engagement level or intervention dosage. All analyses will be carried out using Stata. The results of the power analyses suggest a minimum detectable impact between 0.11 and 0.13, or 11 to 13 percentage points, for the dichotomous outcomes. These figures correspond to a minimum detectable effect size of 0.23 to 0.25 SD, or a reduction in continuous outcome from 50% to 37.5%, assuming a baseline prevalence of 50%.

Prior to estimating any outcome models, the study team will follow WWC group design standards related to baseline equivalence, attrition, and missing data [58]. The team will first test for differences between treatment conditions on baseline measures of all outcomes and demographic factors. Following WWC standards, effect sizes will be computed using Hedge g and any differences above 0.05 noted. Because of random assignment, group differences on baseline or demographic factors are unlikely, but baseline measures for all outcomes will be included as a covariate in all planned impact analyses, and any significant differences in demographic factors (ie, effect sizes between 0.05 and 0.25 SD) will be added as additional covariates.

The team will then calculate overall and differential attrition rates. Overall youth attrition rates will consist of the number of units without observed outcome data in the analysis against the number of units randomized. Differential attrition will be obtained by contrasting the attrition rates between treatment and control groups. Overall youth attrition and differential attrition between the treatment and control groups will be carefully monitored to determine the levels of potential bias.

Missing data for all youth who agree to participate in data collection will be addressed with regression imputation using Stata, with special attention paid to WWC recommendations (eg, separately imputing data from treatment and control conditions). Regression imputation is the preferred approach for this study as it can be used to include participants with missing baseline data and missing outcome data (whereas nonresponse weighting, another imputation method, can only be used for the former). The team will use imputation regression models that (1) are conducted separately by condition or include an indicator variable for condition, (2) include all covariates used for adjustment in the impact model, and (3) include the outcome when imputing missing baseline data.

The study team will use simple averages and frequency distributions to describe the prevalence of each outcome for youth overall and for the treatment group, by selected demographic characteristics and by usage estimates, to provide an initial understanding of changes in the primary and secondary outcomes before estimating impact estimates using multivariate models. The impact of Next4You will be estimated under the ITT framework by comparing outcomes for individuals assigned to Next4You and those assigned to the control condition. The sample will include all youth who participated in the baseline survey and completed the 3-month follow-up, 9-month follow-up, or both. The study team will model all outcomes with a linear-based 2-level multilevel regression model with youth (level 1) nested within level 2 (sites) to account for the nested structure of the data, adjust for correlated error terms, and avoid underestimating standard errors and increasing the probability of type I errors. The following 2-level random intercept multilevel model will estimate the impacts of Next4You on the primary outcome with separate models estimated for the 3- and 9-month follow-ups:

where yij refers to the expected probability of exhibiting the corresponding outcome for individual i in site j; β0 is the overall mean of the outcome across all groups; β1Treat1 ij is an indicator equal to one for participants assigned to the treatment group and zero for those assigned to the control group; β2Baseline2ij is the baseline measure of the outcome; β3X3 ij+…+βpXp ij represents a vector of individual-level baseline covariates (such as sex, race and ethnicity, and age); μj is a site-level random effect; and εij is an individual-level error term. The estimation of the effects of the primary outcome and other dichotomous outcomes via multilevel linear probability models provides a convenient, alternative approach to logit models for interpreting dichotomous outcomes [59]. In particular, linear probability models provide model results on a probability scale that is easier to interpret, whereas logit-based versions of the models express the findings in terms of log odds that are more challenging to interpret.

The use of a 2-level random intercept multilevel model is contingent upon the results of a likelihood-ratio test comparing the fit of the 2-level model against a single-level linear probability model. The null hypothesis for the likelihood-ratio test is that there is no significant difference between the two models. A P value of <.05 for the likelihood ratio test would result in the rejection of the null hypothesis, as there was a statistically significant difference between the models with the 2-level model fitting significantly better than the single-level linear probability model. In the event that the null hypothesis could not be rejected, the study team will use a single-level model.

The team will adjust P values to account for multiple hypothesis testing within each primary and secondary outcome domain using the Benjamini-Hochberg procedure [60]. Finally, the study team will conduct sensitivity analyses to test the robustness of the findings to alternative assumptions or specification of the multilevel models. Sensitivity analyses may involve comparing findings between multilevel logistic regression and multilevel linear probability models, assessing the sensitivity of approaches for addressing missing data, and assessing the impact of sibling participation on outcomes.

User analytics captured directly on the platform will be analyzed to monitor participant engagement with both the intervention and comparison group platforms, exploring user profiles and how engagement may relate to behavioral outcomes in our analyses. Specifically, the study team will use these data to conduct exploratory analyses of dosage and treatment effects as moderators to assess how, if at all, the level of exposure to the platform influences the strength of the observed intervention effect. These data will also be within sensitivity analyses where a “treatment on the treated” analysis framework is used to examine the robustness of the impact estimates under the ITT framework. Various definitions of intervention uptake will be examined, including amount, frequency, duration, and depth among participants who ultimately received the condition to which they were assigned (ie, “compliers”). The effect of treatment receipt for the subgroup of compliers, or the local average treatment effect, will be estimated using instrumental variable methods with a two-stage least squares estimator. In the two-stage least squares, the first stage will involve using the random assignment indicator to predict intervention participation, and the second stage will use the predicted values of intervention participation from the first stage intervention to estimate intervention effectiveness.

This study was funded in 2021 through the Family and Youth Services Bureau as part of the Personal Responsibility Education Innovative Strategies program as an award to ETR. This project was approved by the ETR institutional review board in October 2022. The study is registered at ClinicalTrials.gov (NCT06127277).

All participants provided informed consent or assent prior to participation. Youth aged 16-17 years provided assent under a waiver of parental permission, and youth aged 18-19 years provided informed consent. Consent and assent procedures were conducted electronically and designed to ensure participants’ understanding of study procedures, risks, and benefits, consistent with the fully mobile nature of the intervention. Participants were compensated for their time through electronic gift cards for survey completion, confirmation of contact information, and engagement with the intervention activities. Incentive amounts were structured to reflect participants’ time and effort; full details of compensation and retention strategies are described in the Retention section. All consent forms and survey data will be stored securely within the Qualtrics platform, which employs firewalls, encryption, and independent security audits to protect participant information. Data will be accessible only to authorized project staff using password-protected, two-factor-authenticated devices. Personally identifiable information, such as names or email addresses, will be stored separately from survey responses, and each participant will be assigned a unique study ID used in all analyses. No identifiable information will appear in reports or publications.