Depression and stress have been linked with poor contraceptive behavior, but whether existing mental health symptoms influence women's subsequent risk of unintended pregnancy is unclear. We prospectively examined the effect of depression and stress symptoms on young women's pregnancy risk over one year. We used panel data from a longitudinal study of 992 U.S. women ages 18-20 years who reported a strong desire to avoid pregnancy. Weekly journal surveys measured relationship, contraceptive use and pregnancy outcomes. We examined 27,572 journal surveys from 940 women over the first study year. Our outcome was self-reported pregnancy. At baseline, we assessed moderate/severe depression (CESD-5) and stress (PSS-4) symptoms. We estimated the effect of baseline mental health symptoms on pregnancy risk with discrete-time, mixed-effects, proportional hazard models using logistic regression. At baseline, 24% and 23% of women reported moderate/severe depression and stress symptoms, respectively. Ten percent of young women not intending pregnancy became pregnant during the study. Rates of pregnancy were higher among women with baseline depression (14% versus 9%, p = 0.04) and stress (15% versus 9%, p = 0.03) compared to women without symptoms. In multivariable models, the risk of pregnancy was 1.6 times higher among women with stress symptoms compared to those without stress (aRR 1.6, CI 1.1,2.7). Women with co-occurring stress and depression symptoms had over twice the risk of pregnancy (aRR 2.1, CI 1.1,3.8) compared to those without symptoms. Among women without a prior pregnancy, having co-occurring stress and depression symptoms was the strongest predictor of subsequent pregnancy (aRR 2.3, CI 1.2,4.3), while stress alone was the strongest predictor among women with a prior pregnancy (aRR 3.0, CI 1.1,8.8). Depression symptoms were not independently associated with young women's pregnancy risk. In conclusion, stress, and especially co-occurring stress and depression symptoms, consistently and adversely influenced these young women's risk of unintended pregnancy over one year.