In the United States, up to 14 percent of all pregnancies are affected by gestational diabetes. The prevalence of gestational diabetes has been increasing rapidly over the past few decades (a trend similar to the increase in obesity and type 2 diabetes).

The risk of developing gestational diabetes varies by ethnic background. Women of Native American, Asian, Hispanic, or African American heritage are more likely to be diagnosed with the disease than are non-Hispanic white women.

The causes of gestational diabetes are complex. This condition results from a combination of genetic, health, and lifestyle factors, some of which have not been identified.

A hormone called insulin is important in the development of gestational diabetes. Insulin, which is produced in the pancreas, controls how much glucose (a type of sugar) is passed from the blood into cells to be used as an energy source. Under normal conditions, when blood sugar levels are high (such as after a meal), the pancreas releases insulin to move the excess glucose into cells, which reduces the amount of glucose in the blood.

As a normal part of pregnancy, women develop a reduced ability to respond to the effects of insulin, known as insulin resistance. This process ensures that there is enough glucose to provide energy for the growing fetus. As insulin resistance develops, more and more insulin is needed to keep blood sugar levels within the normal range. As a result, insulin-producing cells in the pancreas (called beta cells) make larger amounts of insulin. These cells are usually able to keep up with the body's demand for insulin, and so most pregnant women do not develop gestational diabetes. However, in some women, pancreatic beta cells are unable to increase production of insulin enough to keep blood sugar levels within the normal range. The result is a rise in blood sugar levels (hyperglycemia) that characterizes gestational diabetes.

Common variations (polymorphisms) in several genes have been associated with the risk of developing gestational diabetes. Because they are common, these variations can be present in people with gestational diabetes and in those without. It is the combination of these changes that helps determine a woman's likelihood of developing the disease. Although little is known about the genetics of gestational diabetes, studies suggest that the genes associated with this form of diabetes overlap with those associated with type 2 diabetes. These genes are involved in the development or function of insulin-producing beta cells in the pancreas or play a role in insulin resistance.

Genetic variations likely act in combination with health and lifestyle factors to influence a woman's overall risk of developing gestational diabetes. Risk factors include having a previous pregnancy affected by gestational diabetes, being older (particularly over age 35) during pregnancy, or having previously had a baby who was large (over 9 pounds) at birth. Other health conditions that predispose to the disease include overweight or obesity, a hormonal imbalance called polycystic ovary syndrome (PCOS), and prediabetes (higher-than-normal blood sugar levels that do not reach the cutoff for diabetes). Many of the risk factors for gestational diabetes are the same factors that increase the risk of developing type 2 diabetes.