Objective: To investigate mothers' changes in prevalence of postpartum depression (PPD) symptoms over 0-9 months postpartum and determine which symptoms best distinguish depressed from nondepressed women. Methods: This was a prospective study of English-literate mothers of newborns, recruited from four family medicine clinics and three pediatric clinics. Mothers completed surveys at 0-1, 2, 4, 6, and 9 months postpartum, and surveys included demographic characteristics, a two-question depression screen, the 9-Item Patient Health Questionnaire (PHQ-9), and other health and work characteristics. Results: There were 506 participants (33% response rate), and 112 (22.1%) had a positive PHQ-9 (score ≥10) at some time within the first 9 months after delivery. The proportion of women with a positive PHQ-9 was greatest at 0-1 month (12.5%), then fell to between 5.0% and 7.1% at 2-6 months, and rose again to 10.2% at 9 months postpartum. Most of the PHQ-9 symptoms differentiated well between depressed and nondepressed women; items that were less discriminating were abnormal sleep, abnormal appetite/eating, and fatigue. Assessment of possible predictors of a change from negative to positive PHQ-9 between 6 and 9 months postpartum revealed only one significant predictor: prior history of depression. Conclusions: Depressive symptoms in this sample were most frequent at 0-1 month and 9 months postpartum. Most PHQ-9 items differentiated well between depressed and nondepressed mothers; these findings support the use of the PHQ-9 for PPD screening. Future research is needed to confirm our observed secondary peak in depressive symptoms at 9 months postpartum and to investigate possible causes.