Low vitamin D status has been associated with unfavourable health outcomes. Postpartum, it is speculated that maternal vitamin D status decreases due to transfer of vitamin D from mother to child through breast milk. A few studies have investigated changes in maternal vitamin D postpartum and possible determinants. Thus, the aims of the present study were to determine changes in serum concentrations of 25-hydroxyvitamin D (25(OH)D) between 2 weeks and 12 months postpartum in Swedish women and to evaluate lactation and other determinants for changes in 25(OH)D concentration postpartum. In total, seventy-eight women were studied at 2 weeks, 4 months and 12 months postpartum. Data collection included measurements of weight and height as well as information about lactation, sun exposure, use of oestrogen contraceptives and physical activity level. Blood samples were collected and serum 25(OH)D levels were analysed using liquid chromatography-tandem MS.