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There are few data focusing on the prevalence of vitamin D deficiency in tropical countries.We determined the vitamin D status in pregnant women and examined the factors associated with vitamin D deficiency.A cross-sectional study of 147 pregnant Thai women aged 18-45 years at Siriraj Hospital (a university hospital in Bangkok, Thailand) was undertaken. Clinical data and plasma levels of 25-hydroxyvitamin D [25(OH)D], intact parathyroid hormone (iPTH), calcium, albumin, phosphate and magnesium were obtained in pregnant women at delivery.The prevalence of hypovitaminosis D [defined as 25(OH)D <75 nmol/L] in pregnant women at delivery was 75.5% (95% confidence interval (CI), 67.7-82.2%). Of these, vitamin D insufficiency [defined as 25(OH)D 50-74.9 nmol/L] was found in 41.5% (95% CI, 33.4-49.9%) and vitamin D deficiency [25(OH)D <50 nmol/L] was found in 34.0% (95% CI, 26.4-42.3%) of women. The mean 25(OH)D concentration was 61.6 ± 19.3 nmol/L. The correlation between 25(OH)D and iPTH was weak (r = -0.29, P<0.01). Factors associated with vitamin D deficiency by multiple logistic regression were: pre-pregnancy body mass index (BMI in kg/m2, odds ratio (OR), 0.88, 95% CI 0.80-0.97, P = 0.01) and season of blood collection (winter vs. rainy, OR, 2.62, 95% CI 1.18-5.85, P = 0.02).Vitamin D deficiency is common among pregnant Thai women. The prevalence of vitamin D deficiency increased in women who had a lower pre-pregnancy BMI and whose blood was collected in the winter. Vitamin D supplementation may need to be implemented as routine antenatal care.