OBJECTIVE: To investigate the association between first-trimester maternal serum 
levels of 25-hydroxyvitamin D (25-OH-D) as measured by liquid 
chromatography-tandem mass spectrometry and development of gestational diabetes 
mellitus (GDM).
RESEARCH DESIGN AND METHODS: We conducted a case-control study involving 248 
women in the first-trimester of pregnancy, 90 of whom developed GDM and 158 
remained normoglycemic.
RESULTS: Although booking 25-OH-D levels correlated negatively with 2-h glucose 
post-oral glucose tolerance test and positively with HDL cholesterol, as well as 
with ethnicity, obesity, and smoking (all P < 0.05), there were no statistically 
significant differences in baseline maternal mean 25-OH-D levels between those 
who subsequently developed GDM, 18.9 ng/mL (SD 10.7) and those who remained 
normoglycemic, 19.0 ng/mL (10.7) (P = 0.874), even after adjustment for possible 
confounders including sampling month (P = 0.784).
CONCLUSIONS: Our large and well-phenotyped prospective study did not find 
evidence of an association between first-trimester maternal levels of 25-OH-D 
and subsequent development of GDM.