Using data from a prospective cohort of 555 adolescent girls and boys from a predominantly Latino neighborhood of San Francisco, we examined how well four survey questionnaire items measuring pregnancy intentions predicted the incidence of pregnancy. We also compared consistency of responses among items and assessed how intentions fluctuated over time. Girls experienced 72 pregnancies over two years (six-month cumulative incidence = 8 percent), and boys reported being responsible for 50 pregnancies (six-month cumulative incidence = 10 percent). Although the probability of becoming pregnant generally increased with higher intention to do so, the risk of becoming pregnant was elevated only at the highest response categories for each item. Most pregnancies occurred among teenagers reporting the lowest levels of intention: for instance, 73 percent of pregnancies occurred among girls who reported that they definitely did not want to become pregnant. Considerable change in respondents' intentions were found over short periods of time: 18 percent and 41 percent of responses to the wantedness and happiness items, respectively, changed between six-month survey visits. The development of appropriate strategies to reduce pregnancy among adolescents would benefit from a more nuanced understanding of how teenagers view the prospect of pregnancy and what determines whether they actively protect themselves from unintended pregnancy.