Background: Postpartum depression (PPD) is the most common complication of childbirth. Suicide is a leading cause of maternal death in the first postpartum year. Depressed mothers often have suicidal ideation (SI). Depression and suicidality may vary across the seasons. Previous studies of seasonality and PPD were relatively small or encumbered by study design constraints. We examined the possible relationship between seasonality, depression, and SI in 9,339 new mothers. Methods: From 2006 to 2010, the investigators screened women within 4–6 weeks postpartum with the Edinburgh Postnatal Depression Scale (EPDS). We used spectral analysis to explore seasonal variation in risk for depression and suicidality. Results: The study team screened 9,339 new mothers, of whom 1,316 (14%) women had positive depression scores (EPDS≥10) which suggest PPD risk; 294 (3%) women had SI (item 10≥1). A positive EPDS was associated significantly with SI. PPD risk varied significantly across 12-months—risk was highest in December. We detected no seasonal variation in SI. Conclusions: Effects of seasonal light variation may contribute to increased risk for depressive symptoms. Suicidality could be related to maternal depression but not seasonal variation. Depression and Anxiety, 2011. © 2011 Wiley-Liss, Inc.