OBJECTIVES: To identify homogenous depressive symptom trajectories over the postpartum period and the demographic and perinatal factors linked to different trajectories. METHODS: Mothers (N = 4866) were recruited for Upstate KIDS, a population-based birth cohort study, and provided assessments of depressive symptoms at 4, 12, 24, and 36 months postpartum. Maternal demographic and perinatal conditions were obtained from vital records and/or maternal report. RESULTS: Four depression trajectories were identified: low-stable (74.7%), characterized by low symptoms at all waves; low-increasing (8.2%), characterized by initially low but increasing symptoms; medium-decreasing (12.6%), characterized by initially moderate but remitting symptoms; and high-persistent (4.5%), characterized by high symptoms at all waves. Compared with the high-persistent group, older mothers (maximum odds ratio [OR] of the 3 comparisons: 1.10; 95% confidence interval [CI]: 1.05 to 1.15) or those with college education (maximum OR: 2.52; 95% CI: 1.36 to 4.68) were more likely to be in all other symptom groups, and mothers who had a history of mood disorder (minimum OR: 0.07; 95% CI: 0.04 to 0.10) or gestational diabetes mellitus diagnosis (minimum OR: 0.23; 95% CI: 0.08 to 0.68) were less likely to be in other symptom groups. Infertility treatment, multiple births, prepregnancy BMI, gestational hypertension, and infant sex were not differentially associated with depressive symptom trajectories. CONCLUSIONS: One-quarter of mothers in a population-based birth cohort had elevated depressive symptoms in 3 years postpartum. Screening for maternal depression beyond the postpartum period may be warranted, particularly after mood and diabetic disorders.