Mothers of preterm infants experience exaggerated emotional stressors compared to those typically associated with new motherhood, making these women particularly vulnerable to postpartum depression. As many as 70% of mothers of preterm infants experience postpartum depression compared to only 12.5% of those delivering full-term infants. Increased stress and depression during this critical period are detrimental because they hamper a mother's ability to care for her infant and are associated with increased neonatal sepsis and mortality, decreased neonatal growth, and delayed motor and cognitive development. Postpartum depression is also associated with excessive maternal weight gain and risk for metabolic diseases, anxiety, and sleep disturbances. Stress in breastfeeding mothers can also alter circulating concentrations of some bioactive components (e.g., immunoglobulins, cortisol) that can transfer into milk. As such, understanding factors predisposing these vulnerable women to extreme levels of stress and finding ways to lower this stress and lessen its negative health outcomes on mothers and infants are important public health challenges. The March of Dimes estimates that 8.5% of births in Idaho are preterm, making this topic particularly relevant for Idaho women. Risk factors for postpartum depression in mothers delivering term or preterm infants are complex, but maternal nutrient deficiencies may be involved. Vitamin D status, for instance, is inversely correlated with risk of postpartum depression in women delivering term infants. However, vitamin D interventions have yielded inconsistent results, perhaps due to confounding impacts of geographic location, skin color, and endogenous vitamin D synthesis. Endogenous vitamin D synthesis requires cutaneous sunlight exposure, placing Idaho women at even greater risk of vitamin D deficiency - particularly in the winter when days are extremely short (only 7 hr on the winter solstice). The impact of maternal vitamin D supplementation during lactation on infant variables (e.g., vitamin D status) has been examined. However, its effect on maternal mental health has not been rigorously studied - let alone in the 'frontier and remote' (FAR) rural West, including Idaho, with short periods of wintertime sunlight and poor access to healthcare. Our long term goal is to develop interventions to improve maternal and infant health in Idaho - particularly in the context of preterm births. The overall primary objective of this proposal is to determine if maternal vitamin D supplementation improves vitamin D status and mental health in Idahoan mothers of preterm infants. Our central hypothesis is that vitamin D supplementation improves vitamin D status and reduces stress and other indicators of poor postpartum maternal mental health in Idaho women delivering preterm infants. Secondarily, we will assess the effects of maternal vitamin D supplementation on human milk composition.