Pregnancy and early motherhood involve uncertainty and change, which can evoke mental health problems. We identified maternal mental health trajectories in pre- and postnatal period, and examined their association with later child mental health and development. Finnish mothers reported psychological distress (General Health Questionnaire [GHQ-36]) and depressive (Beck Depression Inventory [BDI-13]) symptoms in pregnancy (T1; N = 788) and two months (T2; N = 657) and 12 months (T3; N = 545) postpartum. Both parents accounted their child's mental health (Behavior Assessment System for Children [BASC]) and social (Social Skills Rating System [SSRS], Child Behavior Scale [CBS]) and cognitive development (Five to Fifteen [FTF]) when the child was 7-8 years old (T4; N = 485). We identified five trajectories depicting unique timing and course of maternal mental health from pregnancy into 1 year of mothering: Stable low levels of mental health symptoms (75%) and prenatal (6%), early postpartum (9%) and late postpartum (6%) mental health problems. The fifth trajectory, heterogeneous high levels of mental health problems (4%) was an unclassified post hoc class, combining mothers with chronic high or highly variable mental health profiles. Results show some trajectory-related timing effects on children's mental health and cognitive development. The trajectories of early postpartum and heterogeneous high levels of mental health problems predicted higher level of internalizing symptoms as compared to stable low-levels trajectory. The heterogeneous high-levels trajectory predicted higher levels of problems in executive functions than the stable low and late postpartum trajectories, and in memory tasks than children in other trajectories. We discuss the timing and course of maternal mental health from the viewpoint of infant and child development. (Contains 4 tables and 3 figures.)