Several conditions are associated with insulin resistance (IR), and the mechanism behind this state is multifactorial. Pathological states (such as visceral obesity, low energy expenditure, high carbohydrate consumption, and sleep deprivation) and more physiological states (such as puberty and pregnancy) will lead to increased IR. During puberty, the high levels of sex steroids as well as the high levels of growth hormone (GH) are responsible for decreased insulin sensitivity. During pregnancy, the high levels of several diabetogenic hormones lead to a state of IR. A pronounced physiological decrease in peripheral insulin sensitivity occurs as pregnancy proceeds. In contrast, gestational diabetes mellitus (GDM) is a pathological state of carbohydrate intolerance beginning or first recognized during pregnancy, which leads to increased risk of adverse pregnancy outcome. Obese women, an increasing part of the fertile female population, develop a more pronounced IR during pregnancy, with a higher risk of developing GDM.