We are conducting new research to investigate unintended pregnancies during the transition to adulthood. Investigation of these issues requires detailed, timing-specific measures of attitudinal and behavioral aspects of relationships (including sexual behavior), contraceptive use, activities that compete with childbearing (including school and work), and community context. Our investigation also requires detailed, timing-specific measures of beliefs, expectations, and willingness to engage in behaviors related to unintended pregnancy, such as sexual relationships, contraception, pregnancy, and parenthood. Available data resources for studying unintended pregnancy suffer from fundamental weaknesses including retrospective reporting error. In this paper, we describe our new approach to create the prospective measures necessary to study relationships, contraception, and unintended pregnancy among young women. Our approach will provide new estimates of the prevalence of unintended pregnancy among young adults, new insights into the contexts producing unintended pregnancies among young adults, and new measurement strategies. The ongoing project begins with a 60-minute in-person interview about family background and current relationship characteristics. At the conclusion of the interview, respondents are enrolled in an ongoing journal, which consists of a 5-minute survey via web or phone, and occurs weekly for 2.5 years. We have enrolled over 1000 young women in the study and have experienced excellent baseline response rates and high journal participation rates. The current paper describes the study and its design strengths and weaknesses. The results presented throughout the paper are preliminary as data collection ends January 2012.