Background <p>Women with cancer tend to report higher levels of psychological distress than men with cancer. A deeper understanding of how modifiable psychological factors are related to psychological distress in mothers with cancer could inform targeted prevention strategies. The aim of this study was to investigate how the modifiable psychological factors; parenting concerns, self-efficacy, and emotion regulation strategies, are associated with symptoms of depression, anxiety, stress, and posttraumatic stress in mothers with cancer.</p> Methods <p>In 2023, 222 mothers with cancer, aged 25–60 years, with varying time since diagnosis were included in a longitudinal observational study. Data were collected at inclusion and one year later. Outcome measures were symptoms of depression, anxiety, stress, and PTSD, assessed using Depression, Anxiety and Stress Scale (DASS-21) and PTSD Checklist for DSM-5 (PCL-5) questionnaires. Sociodemographic and health-related variables were used as covariates and parenting concerns, self-efficacy, and emotion regulation strategies (cognitive reappraisal and expressive suppression) were used as explanatory variables in the hierarchical linear regression models.</p> Results <p>After controlling for sociodemographic and health-related variables, baseline symptom levels were the strongest predictors of symptoms of depression, anxiety, stress, and PTSD at one-year follow-up. With exception of parenting concerns in relation to depressive symptoms, modifiable psychological factors did not predict the outcomes after adjusting for the contribution of baseline symptoms.</p> Conclusions <p>Baseline symptom severity emerged as a strong predictor across all outcomes, as such early assessment of psychological symptoms may be particularly important for mothers with cancer. Future research should aim to diversify the study population to better capture differences in psychological symptoms across various cancer types, cancer stage, and age groups.</p>