IMPORTANCE: Parental intimate partner violence (IPV) and maternal depression are associated with increased risk of depression in children at the population level. However, it is not known whether having information about these experiences can accurately identify individual children at higher risk of depression. OBJECTIVE: To examine the extent to which experiencing parental IPV and/or maternal depression before age 12 years is associated with depression at age 18 years at the population and individual level. DESIGN, SETTING, AND PARTICIPANTS: This cohort study used data from the Avon Longitudinal Study of Parents and Children, a UK population-based birth cohort, which initially recruited pregnant mothers with estimated due dates in 1991 and 1992. Data used in this study were collected from 1991 to 2009. Data analysis was performed from February to March 2022. EXPOSURES: Mother-reported parental IPV was assessed on 8 occasions (child age, 1-11 years). Maternal depression was assessed via the Edinburgh Postnatal Depression Scale or by the mother taking medication for depression, as reported by the mother on 8 occasions (child age, 2-12 years). MAIN OUTCOMES AND MEASURES: Depressive symptoms were measured with the Short Mood and Feelings Questionnaire (SMFQ) and Clinical Interview Schedule-Revised (CIS-R) when the child was aged 18 years. Binary indicators of a case of depression were derived the cutoff point of 11 points or above for the SMFQ and 12 points or above for the CIS-R. RESULTS: The study included 5029 children (2862 girls [56.9%]; 2167 boys [43.1%]) with a measure of depressive symptoms at age 18 years. IPV only was associated with a 24% (adjusted risk ratio, 1.24; 95% CI, 0.97-1.59) higher risk of depression at age 18 years, exposure to maternal depression only was associated with a 35% (adjusted risk ratio, 1.35; 95% CI, 1.11-1.64) higher risk, and exposure to both IPV and maternal depression was associated with a 68% (adjusted risk ratio, 1.68; 95% CI, 1.34-2.10) higher risk. At the individual level, the area under the receiver operating characteristic curve was 0.58 (95% CI, 0.55-0.60) for depression according to the SMFQ and 0.59 (95% CI, 0.55-0.62) for the CIS-R, indicating a 58% to 59% probability (ie, 8%-9% above chance) that a random participant with depression at age 18 years had been exposed to IPV and/or maternal depression compared with a random participant who did not have depression. CONCLUSIONS AND RELEVANCE: In this cohort study, parental IPV and maternal depression were associated with depression in adolescence at the population level. However, estimation of an individual developing depression in adolescence based only on information about IPV or maternal depression is poor. Screening children for maternal depression and IPV to target interventions to prevent adolescent depression will fail to identify many children who might benefit and may unnecessarily target many others who do not develop depression.