OBJECTIVE: We examined the association of second-trimester maternal plasma 
25-hydroxyvitamin D (25[OH]D) during pregnancy with gestational diabetes 
mellitus (GDM).
STUDY DESIGN: Among 1314 pregnant women who participated in Project Viva, a 
birth cohort study, we measured 25(OH)D levels at 26-28 weeks gestation during 
GDM screening using a 1-hour 50-g glucose challenge test.
RESULTS: We found 25(OH)D levels of <25 nmol/L in 44 of 1087 women (4.0%) with 
normal glucose tolerance, 9 of 159 women (5.7%) with impaired glucose tolerance, 
and 9 of 68 women (13.2%) with GDM. Analyses that were adjusted for 
sociodemographics, season, maternal body mass index, gestational weight gain, 
and dietary factors suggested that women with 25(OH)D levels of <25 vs ≥25 
nmol/L may have higher odds of experiencing GDM (odds ratio, 2.2; 95% confidence 
interval, 0.8-5.5). Glucose levels after the glucose challenge test were 
associated inversely with 25(OH)D levels (P < .01).
CONCLUSION: Second-trimester 25(OH)D levels were associated inversely with 
glucose levels after 1-hour 50-g glucose challenge test; low 25(OH)D levels may 
be associated with increased risk of GDM.