Women with gestational diabetes had significantly lower serum 25-hydroxyvitamin D compared with control subjects (56.3 vs. 62.0 nmol/l, P = 0.018). After adjusting for gestational age and maternal weight, serum 25-hydroxyvitamin D below the top quartile (< 73.5 nmol/l) was associated with a twofold greater likelihood of gestational diabetes (adjusted odds ratio 2.21, 95% confidence interval 1.19-4.13). CONCLUSIONS: Lower vitamin D status in early pregnancy was associated with a significantly increased risk of subsequent gestational diabetes that was independent of race, age, season and maternal weight. This study suggests that vitamin D may influence glucose tolerance during pregnancy