MTNR1B and type 2 diabetes mellitus: Therefore, the difference in allelic effect sizes between GDM and T2D at MTNR1B may reflect the fact that thresholds of fasting plasma glucose used to diagnose GDM are lower than those used to diagnose T2D, meaning that a larger proportion of GDM than T2D cases will have higher fasting glucose that is regulated within the normal range.