BACKGROUND: Evidence is accumulating for a role of vitamin D in maintaining 
normal glucose homeostasis. However, studies that prospectively examined 
circulating concentrations of 25-hydroxyvitamin D (25-[OH] D) in relation to 
diabetes risk are limited. Our objective is to determine the association between 
maternal plasma 25-[OH] D concentrations in early pregnancy and the risk for 
gestational diabetes mellitus (GDM).
METHODS: A nested case-control study was conducted among a prospective cohort of 
953 pregnant women. Among them, 57 incident GDM cases were ascertained and 114 
women who were not diagnosed with GDM were selected as controls. Controls were 
frequency matched to cases for the estimated season of conception of the index 
pregnancy.
RESULTS: Among women who developed GDM, maternal plasma 25-[OH] D concentrations 
at an average of 16 weeks of gestation were significantly lower than controls 
(24.2 vs. 30.1 ng/ml, P<0.001). This difference remained significant (3.62 ng/ml 
lower on average in GDM cases than controls (P value = 0.018)) after the 
adjustment for maternal age, race, family history of diabetes, and pre-pregnancy 
BMI. Approximately 33% of GDM cases, compared with 14% of controls (P<0.001), 
had maternal plasma 25-[OH] D concentrations consistent with a pre-specified 
diagnosis of vitamin D deficiency (<20 ng/ml). After adjustment for the 
aforementioned covariates including BMI, vitamin D deficiency was associated 
with a 2.66-fold (OR (95% CI): 2.66 (1.01-7.02)) increased GDM risk. Moreover, 
each 5 ng/ml decrease in 25-[OH] D concentrations was related to a 1.29-fold 
increase in GDM risk (OR (95% CI): 1.29 (1.05-1.60)). Additional adjustment for 
season and physical activity did not change findings substantially.
CONCLUSIONS: Findings from the present study suggest that maternal vitamin D 
deficiency in early pregnancy is significantly associated with an elevated risk 
for GDM.