The social identity model of identity change provides a useful framework for understanding postpartum depression. Interventions to prevent and treat postpartum depression might aim to support women in maintaining important social group networks throughout pregnancy and the postpartum period.

Objective: We examined the dynamic relationship between life changes (pregnancy and childbirth) and social support during the postpartum period. Methods: A large, nationally representative sample of Australian women (N=806) who completed the Household Income and Labour Dynamics in Australia Survey (HILDA) in the year immediately before and immediately after giving birth to a child reported on measures of perceived social support and mental health. Results: Analyses indicated a decrease in both social support and mental health after having a baby. Social support during the postpartum period – controlling for social support and mental health prior to the birth of a baby – predicted better mental health in women. However, for women who experienced a decline in social support, prenatal social support was a risk factor for a decline in mental wellbeing rather than a protective factor. Conclusions: Women who have ‘more to lose’ are at increased risk of mental ill‐health if they cannot maintain existing sources of social support. Implications for public health: Loss of social support during pregnancy and the postpartum period should be considered as a significant risk factor for postpartum depression in its own right and one that warrants screening and intervention.