During pregnancy, a number of maternal metabolic changes occur early and continue throughout pregnancy which help optimize the transfer of nutrients to the fetus. During normal pregnancy, there are a decrease in insulin sensibility which is physiological, progressive and reverse. For glucose tolerance to be maintained in pregnancy it is necessary for maternal insulin secretion to increase sufficiently to counteract the fall in insulin sensitivity. The metabolic characteristic of women with gestational diabetes is insufficient insulin secretion to counteract the pregnancy related fall in insulin sensitivity. There are a lot of factors that could explain the mechanism of insulin secretion and insulin sensitivity during normal pregnancy and gestational diabetes mellitus. Although glucose tolerance normalizes shortly after pregnancy with gestational diabetes in the majority of women, the risk of developing overt diabetes, especially type 2 diabetes is markedly increased. The mechanisms which could explain gestational diabetes are the same as type 2 diabetes mellitus. We could speculate that these two diseases are identical for alterations in carbohydrate metabolism, but at different stages.