BACKGROUND: The biology of peripartum depression remains unclear, with altered stress and the Hypothalamus-Pituitary-Adrenal axis response having been implicated in its pathophysiology.METHODS: The current study was undertaken as a part of the BASIC project (Biology, Affect, Stress, Imaging, Cognition), a population-based longitudinal study of psychological wellbeing during pregnancy and the postpartum period in Uppsala County, Sweden, in order to assess the association between evening salivary cortisol levels and depressive symptoms in the peripartum period. Three hundred and sixty-five pregnant women from the BASIC cohort were recruited at pregnancy week 18 and instructed to complete a Swedish validated version of the Edinburgh Postnatal Depression Scale at the 36th week of pregnancy as well as the sixth week after delivery. At both times, they were also asked to provide evening salivary samples for cortisol analysis. A comprehensive review of the relevant literature is also provided.RESULTS: Women with postpartum EPDS score ≥ 10 had higher salivary evening cortisol at six weeks postpartum compared to healthy controls (median cortisol 1.19 vs 0.89 nmol/L). A logistic regression model showed a positive association between cortisol levels and depressive symptoms postpartum (OR = 4.1; 95% CI 1.7-9.7). This association remained significant even after controlling for history of depression, use of tobacco, partner support, breastfeeding, stressful life events, and sleep problems, as possible confounders (aOR = 4.5; 95% CI 1.5-14.1). Additionally, women with postpartum depressive symptoms had higher postpartum cortisol levels compared to both women with depressive symptoms antenatally and controls (p = 0.019 and p = 0.004, respectively).CONCLUSIONS: Women with depressive symptoms postpartum had higher postpartum cortisol levels, indicating an altered response of the HPA-axis in postpartum depression.