Becoming a parent is a transformative experience, marked by hormonal changes and neuroplasticity as well as shifts in self-concept, social roles, and daily routines. Although the arrival of a new baby is often a joyful event, the postpartum period can also be a time of heightened psychosocial stress and health behavior changes, including significant sleep disruption and decreased physical activity. Markers of allostatic load, such as physiological stress and inflammation, may also become dysregulated during this time. Given these neurobiological, psychosocial, and behavioral changes, the transition to parenthood may shape health trajectories in midlife. For many mothers and fathers, the transition to parenthood represents an inflection point for obesity, such that perinatal weight gains are retained long-term. Similarly, many individuals experience their 1st episode of major depression during the postpartum period. In sum, the transition to parenthood may represent a critical window for determining both mental and physical health in midlife and beyond. Physical and mental health problems over the transition to parenthood may be exacerbated for parents without access to protected, paid time off from employment. Known disparities in mood disorders, obesity, and allostatic load may be linked to risk factors stemming from the perinatal period. This article relates the importance of the transition to parenthood to population health and discusses parental leave policy as an example of an initiative that can support parents and relieve stress during the perinatal period. (PsycINFO Database Record (c) 2018 APA, all rights reserved).