Background Social concern with teen pregnancy emerged in the 1970s, and today’s popular and professional health literature continues to draw on social norms that view teen pregnancy as a problem—for the teen mother, her baby, and society. It is unclear, however, how adolescents directly affected by teen pregnancy draw upon social norms against teen pregnancy in their own lives, whether the norms operate differently for girls and boys, and how these social norms affect pregnant or parenting adolescents. Objective This research aims to examine whether and how US adolescents use, interpret, and experience social norms against teen pregnancy. Methods Online ethnographic methods were used for the analysis of peer-to-peer exchanges from an online social network site designed for adolescents. Data were collected between March 2010 and February 2015 (n=1662). Thematic analysis was conducted using NVivo software. Results American adolescents in this online platform draw on dominant social norms against teen pregnancy to provide rationales for why pregnancy in adolescence is wrong or should be avoided. Rationales range from potential socioeconomic harms to life-course rationales that view adolescence as a special, carefree period in life. Despite joint contributions from males and females to a pregnancy, it is primarily females who report pregnancy-related concerns, including experiences of bullying, social isolation, and fear. Conclusions Peer exchange in this online forum indicates that American adolescents reproduce prevailing US social norms of viewing teen pregnancy as a social problem. These norms intersect with the norms of age, gender, and female sexuality. Female adolescents who transgress these norms experience bullying, shame, and stigma. Health professionals must ensure that strategies designed to prevent unintended adolescent pregnancy do not simultaneously create hardship and stigma in the lives of young women who are pregnant and parent their children.