Vitamin D deficiency is common in pregnancy, especially in obese women. Lifestyle intervention could potentially result in higher levels of Vitamin D. We therefore aimed to study the effect of lifestyle intervention during pregnancy on serum levels of 25-hydroxyvitamin D (25(OH)D).360 obese women were randomised before gestational age 14 weeks to lifestyle intervention (diet and exercise) or routine clinical follow-up (controls). Clinical outcomes and levels of 25(OH)D were determined three times: At gestational age 12-15 weeks (baseline), gestational age 28-30 weeks and six months postpartum.A total of 304 (84%) women completed the intervention study, and 238 (66%) attended postpartum follow-up. Vitamin D levels were similar in the two groups at baseline. At gestational age 28-30 weeks and 6 months postpartum, 25(OH)D levels were significantly higher in the intervention group compared to controls (75.6 vs 66.8 nmol/L, P = 0.009) and (54.8 vs 43.1 nmol/L, P = 0.013), respectively. Concurrently, vitamin D deficiency (25(OH)D <50 nmol/L) was less frequent in the intervention group compared to controls: 15% vs. 25% (P = 0.038) at gestational age 28-30 and 45% vs. 63% (P = 0.011) six months postpartum, respectively.Lifestyle intervention during pregnancy was associated with significantly increased vitamin D levels in late pregnancy and postpartum compared to controls. This article is protected by copyright. All rights reserved.