Study Question: What is the predictive value of pregnancy intentions on contraceptive behaviours among women aged 18-19? Summary Answer: Women aged 18-19 have high levels of inconsistent use of contraception, which mostly occur at times when women strongly wish to avoid a pregnancy. What is Known Already: Pregnancy intentions provide an indication of how well individuals achieve their reproductive goals. However, retrospective accounts of pregnancy intentions using dichotomous indicators suffer temporal instability and fail to capture the wide range of attitudes towards pregnancy. Study Design , Size, Duration In this study, data are drawn from a population-based survey of 992 women of ages 18-19 years in Michigan, who completed weekly journals assessing contraceptive use, pregnancy intentions and reproductive outcomes during 2.5 years of follow-up. The response rate was 86% for the baseline interview and 65% after 2.5 years of follow-up. Participants/Materials, Setting, Methods We examined 15 446 pairs of journal entries. We used logistic regression with random effects to assess the predictive effect of women's desire to become pregnant and to avoid a pregnancy, measured each week, on consistency of use of contraception the following week. Main Results and the Role of Chance: Women reported inconsistent use of contraception in more than a quarter of weekly journals (28.3%). Consistent use of contraception increased from 22 to 78% as women s intentions to become pregnant decreased and increased from 23 to 78% as motivations to avoid pregnancy increased. The combination of scores of the pregnancy desire and avoidance scales shows indifferent or ambivalent pregnancy attitudes in 8.6% of weekly records. These women were more likely to report inconsistent contraceptive use compared with women who expressed anti-conception attitudes [OR = 2.8 (2.2-3.5)]. However, 23% of women who had unequivocal anti-conception feelings did not use contraception consistently, contributing to 72% of the weeks of inconsistent use in our population. Limitations, Reasons For Caution In this study, consistency of contraceptive use, based on the use of contraception at every act of intercourse, does not fully capture a women's risk of becoming pregnant. The 35% attrition after 2.5 years may have affected the internal validity of our results, although a reanalysis based on the first year of observation produced very similar results. Wider Implications of the Finding: s: Because most instances of inconsistent use of contraception occur among women who are keen to avoid a pregnancy, our results suggest there is room for improving contraceptive behaviours by promoting use of methods which do not require user adherence.