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Prenatal tobacco exposure (PTE) and tobacco–cannabis coexposure (PTCE) co-occur with negative maternal emotional functioning (termed prenatal risks) and together increase risk for child regulatory problems at early school age (ESA). Little is known about developmental processes in early childhood that may mediate this association. We examined two hypothesized mediational processes linking prenatal risks to ESA emotion regulation (ER) and lability–negativity; parasympathetic functioning at toddler age and chronic risk reflected by continued postnatal maternal negative emotional functioning (i.e., depression, anger/hostility, and emotion dysregulation) and substance exposure. Congruent with differential susceptibility theory, we examined interactions between sensitive parenting and toddler parasympathetic functioning predicting ESA ER. Finally, we explored the role of child sex as a moderator. Mothers (N = 247; 53% male infants; 51% Black, 31% White, 19% Hispanic, and 8% other or mixed race) were recruited in the first trimester of pregnancy into one of three groups: PTE (n = 81), PTCE (n = 97), and no substance exposure (n = 69) matched on age and education. Substance exposure was assessed using multiple methods, maternal negative emotional functioning via self-reports, parenting with observations, and child ER using teacher, maternal, and lab assessor reports. Results supported a chronic risk pathway with less support for a parasympathetic pathway. Toddlers who demonstrated respiratory sinus arrhythmia withdrawal to frustration were susceptible to the positive context of...