The aim of this secondary analysis was to determine whether seasonal light exposure, categorized by type of day length, is associated with or predictive of depressive symptoms in late pregnancy and the first 3 months postpartum. Women (n = 279) expecting their first child were recruited from prenatal clinics and childbirth education classes. Depressive symptoms were assessed with the Center for Epidemiologic Studies Depression Scale. Day lengths were categorized into short, lengthening, long and shortening. Data analysis included linear mixed models and multiple linear regression. When days were shortening (August to first 4 days of November) in late third trimester, depressive symptom scores were highest (35%) and continued to be higher at each postpartum assessment compared to other day length categories. Implications for clinical practice include increased vigilance for depressive symptoms, particularly if late pregnancy and birth occurs during the 3 months around the Autumn equinox when day length is shortening. Strategies that increase light exposure in late pregnancy and postpartum should also be considered.