Comparison of seasonal serum 25-hydroxyvitamin D concentrations among pregnant women in Mongolia and Boston.

•Vitamin D levels in Mongolian third trimester pregnant women are severely depressed.•Levels in pregnant women in Boston are comparatively elevated and less seasonal.•Levels in Boston, but not Mongolia, are associated with targetable risk factors.•Mongolia should consider a combined approach of fortification and supplementation.•This joint approach is warranted in vitamin D deficiency-endemic regions globally.