Conventional treatment for gestational diabetes mellitus increases the proportion of infants born with a low birth weight, a risk factor for cardiovascular disease and diabetes mellitus in later life. Thiamine supplementation during pregnancy may be shown to be a safe preventive measure. During pregnancy, approximately 50% of the women develop a biochemical thiamine deficiency, whereas the thiamine status falls, but remains within normal limits, in most other women. Thiamine is essential for glucose oxidation, insulin production by pancreatic beta-cells and cell growth. It is therefore likely that thiamine supplementation in pregnant women not only improves their glucose tolerance but also stimulates the intra-uterine growth, thereby preventing a low birth weight to ensue from conventional therapy which only improves glucose tolerance.