Potential biologic and psychosocial causative factors for the postpartum blues were tested in a prospective study of 182 women followed up from the second trimester of pregnancy until postpartum week 9. Personal and family history of depression, depressive symptoms, stressful life events, and social adjustment were all assessed during the second trimester. Levels of progesterone, prolactin, estradiol, free and total estriol, and free and total cortisol were measured on several occasions during late pregnancy and early puerperium. Obstetric and child-care stressors and the postpartum blues were assessed after delivery. Predictors of the postpartum blues were personal and family history of depression, social adjustment, stressful life events, and levels of free and total estriol. Our results support the hypothesis that the postpartum blues is within the spectrum of affective disorders.