Background: Gaining more weight during pregnancy than is recommended by the Institute of Medicine is prevalent and contributes to the development of obesity in women. This article describes the development and use of e-Moms of Rochester (eMoms Roc), an electronic intervention (e-intervention), to address this health issue in a socioeconomically diverse sample of pregnant women. Materials and Methods: Formative research in the form of intercept interviews, in-depth interviews, and focus groups was conducted to inform the design of the e-intervention. The Web site continuously tracked each participant's use of e-intervention features. Results: An e-intervention, including Web site and mobile phone components, was developed and implemented in a randomized control trial. Formative research informed the design. Participants in all arms accessed blogs, local resources, articles, frequently asked questions, and events. Participants in the intervention arms also accessed the weight gain tracker and diet and physical activity goal-setting tools. Overall, 80% of women logged into the Web site and used a tool or feature at least twice. Among those in the intervention arm, 70% used the weight gain tracker, but only 40% used the diet and physical activity goal-setting tools. Conclusions: To maximize and sustain potential usage of e-Moms Roc over time, the e-intervention included customized reminders, tailored content, and community features such as blogs and resources. Usage was comparable to those in other weight studies with young adults and higher than reported in a published study with pregnant women. This e-intervention specifically designed for pregnant women was used by the majority of women.