Purpose <p>The goal of this study was to understand how the timing, level, and fluctuation of pregnancy stress affected women’s postpartum mental health while also considering their experiences of early life stress (adverse childhood events; ACEs). We hypothesized that overall stress level, as well as stress during particular pregnancy weeks, would influence postpartum mental health. We further hypothesized that fluctuations in weekly perceived stress levels across pregnancy would influence postpartum mental health among women who had experienced ≥ 4 ACEs.</p> Methods <p>We collected information on ACEs and weekly stress data from our participants during pregnancy (15 weeks until parturition). Women’s depressive and anxiety symptoms were assessed at six-months postpartum. We employed a machine learning approach to explore the predictive utility of timing, level, and fluctuation of pregnancy stress on postpartum mental health symptoms.</p> Results <p>For all women, higher average stress level across all weeks of pregnancy predicted greater postpartum depressive and anxiety symptoms. Additionally, women with a history of ≥ 4 ACEs were susceptible to stress levels in weeks 20 of pregnancy (depressive symptoms only) and 26 (depressive and anxiety symptoms). Lastly, greater variability in stress levels across pregnancy was also associated with greater depressive symptoms for the women with ≥ 4 ACES.</p> Conclusion <p>In addition to overall stress level, timing of pregnancy stress and fluctuations in stress levels across pregnancy are associated with postpartum mental health for women with a history of substantial ACEs, with implications for focused mental health interventions during pregnancy for these women.</p>