CONTEXT: In pregnancy, vitamin D insufficiency and deficiency, defined as serum 25-hydroxyvitamin D (25(OH)D) <50 nM, and <25 nM, respectively, may have adverse effects for both mother and child. Prevalence estimates, and identification of subgroups at special risk, may be useful for the planning of preventive strategies. OBJECTIVE: To study the prevalence and risk factors of hypovitaminosis D in early pregnancy. Design and Methods: In a cross-sectional study of 1,348 women in early pregnancy from the Odense Child Cohort, Denmark, 25(OH)D was determined and correlated to demographic and lifestyle variables (age, nationality, skin tone, parity, pre-pregnancy body mass index (BMI), smoking, and sun exposure), using multiple linear and logistic regression analyses for all-year, or stratified for summer and winter. The risk of vitamin D insufficiency was expressed as odds ratios (OR) with 95% confidence intervals in brackets. RESULTS: The prevalence of vitamin D insufficiency and deficiency was estimated to 27.8%, and 3.5%, respectively. In adjusted analyses, vitamin D insufficiency was directly associated with winter season, OR=1.89 (1.35 - 2.63); increasing pre-pregnancy BMI, OR=1.06 (1.03-1.10); and smoking, OR=2.7 (1.34-5.41); but was less frequent in nulliparous, OR=0.47 (0.33-0.68) and tanned Caucasians, OR=0.63 (0.41-0.97). Season-specific associations were parental origin from outside Europe in summer, OR=4.13 (1.41-12.13); in winter smoking, OR=3.15 (1.19-8.36); and pre-pregnancy BMI, OR=1.12 (1.06-1.18). CONCLUSIONS: Vitamin D insufficiency was widespread in early pregnancy. Associations to smoking, pre-pregnancy BMI, and origin outside Europe varied with season. Multiparity and not being tanned in Caucasians represent new risk factors of vitamin D insufficiency. © 2013 Blackwell Publishing Ltd.