What Causes Gestational Diabetes?

Gestational diabetes is a kind of diabetes that can only be found in pregnancy. When a woman has gestational diabetes, the fasting blood sugar level may be normal but testing will reveal that the woman’s cells and tissues are resistant to insulin, resulting in high blood sugars, particularly after eating. This high blood sugar state is usually completely asymptomatic but can have serious consequences for the fetus in utero and for the baby after birth. This is the main reason why gestational diabetes needs to be treated in all women having the disorder.

Risk Factors for Gestational Diabetes

Some amount of insulin resistance, mildly elevated blood sugars, and cellular impairment of glucose tolerance is a normal finding during late pregnancy.  In some women, however, this impaired glucose tolerance rises to the level of gestational diabetes, needing the management of the OB/GYN doctor, a diabetic specialist, and a dietician during the pregnancy.

The main risk factors for gestational diabetes include the following:

• Having gestational diabetes in a prior pregnancy
• Being obese during this pregnancy or prior to becoming pregnant
• Having a diagnosis of pre-diabetes before the pregnancy
• Having a personal history of polycystic ovarian syndrome prior to the pregnancy
• Delivering a child in a previous pregnancy that weighed more than 9 pounds
• Having a first degree relative with type 2 diabetes mellitus
• Being African American, Hispanic ethnicity, Asian American, American Indian, or Pacific Islander ethnicity

Causes of Gestational Diabetes

No one knows exactly why some women develop gestational diabetes and why some women don’t—even if they have the same risk factors.  The causes of gestational diabetes can be best understood by taking a look at how glucose is processed in the body.

Normally, the body digests the carbohydrates and other macronutrients from the food you eat and sugar is produced in the digestive process.  This sugar is mainly glucose, which is the main cellular fuel used by the cell for normal cell metabolism.  The pancreas, which is the large gland located in the middle of the upper abdomen behind the stomach, has cellular tissue called the Islets of Langerhans that respond to the glucose circulating in the blood stream.

The Islets of Langerhans produce insulin in response to the amount of glucose in the bloodstream by releasing insulin.  Ideally, the amount of insulin produced is exactly the amount necessary to bind to the cells of the body to allow the glucose to enter the cells.

During pregnancy, the placenta (which provides the blood supply to the fetus) produces a large number of hormones. Nearly all of these hormones will block the action of insulin in the cells, making the cells resistant to the insulin.  They may block insulin receptors or may allow insulin to bind to the receptors but may render the insulin unable to allow the intake of glucose into the cells.  The end result is that even more insulin is put out by the pancreas (which won’t work to put glucose into the cells) and the average blood sugar increases.

This means that all women in pregnancy will have mild elevations in blood glucose values after eating but some women will have a greater degree of insulin resistance that rises to the level of gestational diabetes.  The placental hormones remain elevated throughout pregnancy and increase as the pregnancy continues.  At some point, usually between the twentieth week of pregnancy to the twenty-fourth week of pregnancy, the placental hormones are so high that insulin fails to bring the blood sugar into the normal range after a meal and gestational diabetes ensues.

Some researchers believe that the pancreas in pregnancy doesn’t produce enough insulin during the latter half of the pregnancy so that there isn’t enough to put all of the glucose absorbed by the gastrointestinal tract and the glucose level remains elevated, particularly after a high carbohydrate or high sugar meal.

Weight gain during pregnancy may also contribute to gestational diabetes.  It is well known that women who are obese or women who gain a lot of weight in the first half of pregnancy will be at a greater risk of gestational diabetes during pregnancy.  Fat cells release hormones that also cause insulin resistance nearly as much as the hormones released by the placenta.  This would explain why obese women in pregnancy have a higher chance of having gestational diabetes.

Decreasing the Incidence of Gestational Diabetes

Women at risk for gestational diabetes because of risk factors, particularly those who have had gestational diabetes in the past, can do a few things to reduce their chances of getting gestational diabetes. Losing weight before getting pregnant will decrease the slight insulin resistance that already exists in women who are obese when they become pregnant. Increasing one’s physical activity before pregnancy will not only help weight loss but will positively affect the woman’s insulin sensitivity so that the risk of gestational diabetes during the pregnancy will be less.

Complications of Gestational Diabetes

Women are screened for gestational diabetes at about twenty-four weeks’ gestation, which is the time when most women will begin to show evidence of having insulin resistance and elevated blood sugars after drinking a 50-gram load of glucose solution.  Women who fail this screening test will have a confirmatory 3-hour glucose tolerance test and those who fail this will be labeled as having gestational diabetes.

The goal of treatment for gestational diabetes is to increase physical activity and reduce heavy glucose loads when eating.  A dietician or nutritionist will be able to help the woman with lifestyle changes that may avoid the complications of the disorder.

Women who don’t have adequate treatment for gestational diabetes will have ongoing and worsening insulin resistance and will have persistently high blood glucose, especially after carbohydrate-containing meals.  Because glucose crosses the placental barrier, the fetus will also have elevated blood glucose levels as well and will put out excessive amounts of insulin.  This will result in a fetus that grows too large and that has an excessive amount of fat on its body.

The end result of untreated gestational diabetes is an infant that is large for gestational age or “macrosomia”, which causes difficulties having a vaginal birth. The infant will have hypoglycemia right after birth that may require IV glucose solution and will have a higher chance of dying in utero or immediately after birth.  Babies who live can have respiratory distress syndrome that may need intensive care treatment right after birth.