The onset of parenthood has been reported as a reason for steep declines in moderate-vigorous intensity physical activity (MVPA), but also increases in light activity rather than sedentary behavior. We examined the activity profiles of three cohorts of couples (couples without children, and first-time parents and second time parents) across 12 months. Participants were 314 adults (102 not expecting a child, 136 expecting first-child, 76 expecting second child) who completed baseline demographics and 7-day accelerometry, followed by assessments at 6 and 12 months. Hierarchical linear modeling showed that parents who were expecting their second child had lower MVPA; yet were less sedentary/had higher light intensity activity compared to other couples at baseline. First-time mothers' physical activity pattern changed to match the profiles of parents who were now parenting two children across the first 12 months of child-rearing. Findings support MVPA interventions targeting new mothers.