Since thiamin plays a role in glucose metabolism we wanted to know if blood thiamin influx from gravida to neonate was influenced by treatment of gravidas having gestational diabetes mellitus (GDM). In this study we found thiamin hypovitaminemia in 19% of the 77 pregnancies despite vitamin supplementation and treatment for GDM; neonates born to mothers with hypovitaminemia were also thiamin hypovitaminemic. All neonatal blood had significantly higher thiamin concentration than gravidas. Indeed, cord blood from neonates born to mothers treated with insulin for GDM had significantly higher thiamin concentration than other neonates in the study. A significant weight depression was noted in neonates born to treated GDM mothers. Healthy gravidas giving birth to macrosomia neonates, had significant thiamin hypovitaminosis, but only macrosomic neonates of treated diabetic mothers had significantly depressed blood thiamin concentrations. We noted that subclinical thiamin hypovitaminemia is prominent during pregnancy despite vitamin supplementation. Perhaps increased thiamin supplementation during pregnancy seems warranted to avoid metabolic stress in mother and fetus due to thiamin hypovitaminemia.