PURPOSE: This prospective case-cohort study compared subjective well-being (SWB) among mothers whose children had various degrees of congenital heart defects (CHD) with mothers of children without CHD (controls). METHODS: Nationwide CHD registry data were linked to data collected from the Norwegian Mother and Child Cohort Study at gestational week 30, 6 months, and 36 months postpartum. A total of 175 mothers of children with mild, moderate, and severe CHD were identified in a cohort of 44,144 mothers. The SWB index was operationalized by means of three subscales: a cognitive aspect, positive affect, and negative affect. RESULTS: Mothers of children with severe CHD reported significantly lower SWB than the controls at 6 months postpartum (p = .003), with further decrease in SWB at 36 months postpartum (p = .001). SWB levels in the mild and moderate CHD group did not deviate significantly from controls. CONCLUSIONS: The findings suggest a pattern in which all stressors concerning the severely ill child lead to significant deterioration of maternal well-being. Having a child with mild or moderate CHD, which is a less severe and shorter-term stressor, did not reduce mothers' well-being.