BACKGROUND: Vitamin D deficiency may contribute to impaired glucose metabolism. 
There are sparse data regarding vitamin D and the development of gestational 
diabetes (GDM). The objective of this study was to assess if first-trimester 
vitamin D deficiency is more prevalent in women later diagnosed with GDM 
compared with women with uncomplicated pregnancies.
METHODS: We conducted a nested case-control study of pregnant women who had 
previously given blood for routine genetic multiple marker screening and 
subsequently delivered at a tertiary hospital between November 2004 and July 
2009. From an overall cohort of 4225 women, 60 cases of GDM were matched by 
race/ethnicity with 120 women delivering at term (≥37 weeks) with uncomplicated 
pregnancies. Banked maternal serum was used to measure maternal 
25-hydroxyvitamin D [25(OH)D].
RESULTS: The prevalence of first-trimester maternal vitamin D deficiency 
(defined as 25(OH)D < 50 nmol/L) was comparable among women with GDM compared 
with controls (5/60 vs 8/120, p = 0.90). The median 25(OH)D level for all 
subjects was 89 nmol/L (interquartile range, 73-106 nmol/L). Seventy three 
percent (117/160) of the cohort had 25(OH)D levels ≥75 nmol/L.
CONCLUSIONS: In a cohort of pregnant women with mostly sufficient levels of 
serum 25(OH)D, vitamin D deficiency was not associated with GDM. Further studies 
are warranted with larger cohorts, especially in populations with lower levels 
of vitamin D.