The first positive genome-wide association study on gestational length and preterm delivery showed associations with a gene involved in the selenium metabolism. In this study we examine the associations between maternal intake of selenium and selenium status with gestational length and preterm delivery in 72,025 women with singleton live births from the population based, prospective Norwegian Mother, Father and Child Cohort Study (MoBa). A self-reported, semi-quantitative food-frequency questionnaire answered in pregnancy week 22 was used to estimate selenium intake during the first half of pregnancy. Associations were analysed with adjusted linear and cox regressions. Selenium status was assessed in whole blood collected in gestational week 17 (n=2,637). Median dietary selenium intake was 53 (IQR: 44-62) µg/day, supplements provided additionally 50 (30-75) µg/day for supplement-users (n=23,409). Maternal dietary selenium intake was significantly associated with prolonged gestational length (β per SD=0.25, 95% CI=0.07-0.43) and decreased risk for preterm delivery (n=3,618, HR per SD=0.92, 95% CI=0.87-0.98). Neither selenium intake from supplements nor maternal blood selenium status was associated with gestational length or preterm delivery. Hence, this study showed that maternal dietary selenium intake, but not intake of selenium containing supplements, during the first half of pregnancy was significantly associated with decreased risk for preterm delivery. Further investigations, preferably in the form of a large RCT, are needed to elucidate the impact of selenium on pregnancy duration.