As has been shown in the previous chapters, major changes take place in the intermediary metabolism throughout pregnancy: changes that will facilitate the fetal needs of energy and precursors for fetal and placental growth, as well as for placental hormone synthesis. From a metabolic point of view, during gestation there are two different periods. In the first half of pregnancy, during the embryo development period, there are maternal changes that lead to storage of energy and nutrients. 1 3 During this time of pregnancy, there is both increased appetite and normal or increased insulin sensitivity. These changes will facilitate glucose and lipid uptake by adipose tissue, increasing the lipid stores. 1 3 In fact, during the first half of pregnancy, most women show an increase in adipose tissue mass. 4 Nevertheless, as pregnancy advances and the fetal-placental unit is rapidly growing, a marked shift in the metabolic pathways is observed. This second period is characterized by a state of maternal insulin resistance, decreasing the uptake of glucose in the insulin-sensitive tissues, mainly the white adipose tissue and muscle (Figure 4.1, panel b). Such a condition facilitates the supply of glucose toward the fetus, where the daily glucose requirements are very high (30–50 g of glucose/day). 5 The insulin resistance is also responsible of the blunted curve that takes place after an oral glucose tolerance test or a regular meal. 6 8 Such a curve causes higher and more prolonged plasma levels of glucose after a meal facilitating a higher glucose supply toward the fetus, as the transfer of glucose through the placenta is via passive glucose diffusion and, therefore, concentration dependent. 9 Intermediary metabolism in nonpregnant women (panel a), normal pregnancies (panel b), and pregnancies with GDM (panel c). The thickness of the lines relates to the degree of stimulation (solid lines) or inhibition (dash lines) for both the metabolic pathways and the hormonal effects. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315382104/214c229b-14ed-4e52-a0d1-fb31003975c1/content/fig4_1.tif"/>