What is Gestational Diabetes?


Diabetes is a disease in which the body cannot use food for energy correctly. The most common forms of diabetes are called type 1 and type 2. A third type, called gestational diabetes (GDM), is diabetes that develops or is first recognized during pregnancy. When a woman has GDM, she loses the ability to control the level of sugar in her blood automatically.

Our bodies use glucose (a form of sugar) from the food we eat to produce energy. The body needs insulin to move sugar from the bloodstream into body cells. That is where the sugar is used. A gland called the pancreas makes insulin. Most women who develop GDM had normal glucose levels before pregnancy and will return to that state when the baby is born. They are still making insulin but the insulin they are making does not work as it should. A genetic predisposition to this condition combines with the many hormones produced by the placenta during pregnancy, making the woman's body resistant to the action of its own insulin. Without enough insulin on the job, sugar stays in the blood. Blood sugar levels then rise above normal. This is an unsafe situation for both mother and baby.

Some women who are diagnosed as having GDM actually have either Type 1 or Type 2 diabetes that had not been diagnosed before the pregnancy began. This can happen if the woman developed one of these permanent forms of diabetes before or during pregnancy that is then discovered during routine screening for GDM. So, there is a remote possibility that a woman who is found to have high blood sugars during pregnancy may actually have Type 1 or Type 2 rather than GDM. That is one reason why all women with GDM should have a follow-up blood sugar test performed about 6 weeks following delivery. This is a safety measure to make sure that blood sugar has returned to normal.

When GDM is well controlled, the mother should deliver a normal healthy baby, although both mother and child have higher risk for certain future health problems (see Look to the Future below).

Uncontrolled GDM is associated with higher risk of both illness and death for the infant, especially in the period surrounding birth. These problems include large fetal size due to babies receiving too much glucose from the mothers' blood during the later months of gestation, immaturity when babies are delivered before the due date due to size, respiratory distress, and low blood glucose in the baby after delivery. Birth defects and spontaneous abortion are seldom seen in GDM because hyperglycemia typically develops later in the pregnancy, after the development of organs is complete.

Who Gets Gestational Diabetes?

About 4% of all pregnancies in the US are complicated by GDM. Rates of GDM are higher among women with:

The prevalence of GDM in certain high-risk groups can be as high as 14%.

Screening for Gestational Diabetes

Most women should be screened at 24-26 weeks gestation (end of second trimester). Rising placental hormones trigger the insulin resistance that leads to GDM. Levels of these hormones continue to rise over the course of pregnancy. Therefore, GDM tends to occur later in pregnancy as a rule.

Women at high risk (those who are obese, have a personal history of GDM, or family history of diabetes) or have sugar in the urine should be screened earlier, by the end of the first trimester. If the first test is negative in a woman at high risk, the screening test should be repeated again at 24-26 weeks gestation.

Screening is likely to find very few cases among women who meet all of the following criteria and may be skipped at the provider's discretion:

Screening consists of a plasma glucose test performed one hour after consuming 50 grams of glucose. If the test is performed after an overnight fast, a one-hour glucose value of 140 mg/dl or greater is considered to be a positive screen. If the test is performed after a meal, the screening threshold is reduced to 130 mg/dl.

Gestational diabetes is diagnosed when:

If the screening result is above the appropriate threshold number but below 198 mg/dl, a full 100-gram, 3-hour glucose tolerance test is done to determine the diagnosis. The following benchmarks are used:

A woman with even one abnormal value on the glucose tolerance test should receive dietary advice (described below) and instruction in blood glucose monitoring in order to perform occasional post meal blood tests. The glucose tolerance test should be repeated in 4 weeks.


The goal of treatment in GDM is to keep maternal blood sugar in the normal range. This is the best way to prevent complications for the baby and a difficult delivery. In order to achieve these goals:

The first line of therapy is nutritional management. Women with GDM should work with a registered dietitian familiar with diabetes. Gestational diabetes does not change the special nutritional needs of pregnancy. Except in rare cases of severe obesity where the obesity itself is a risk, limiting overall food intake control blood glucose is not recommended. Rather, working with a knowledgeable professional, and based on the outcome of blood glucose testing, the woman with GDM may need to make certain substitutions or move carbohydrate-containing foods around in the day to achieve glucose goals. It is usually helpful for women with GDM to eat six small meals per day to assist with glucose control, including protein and fat in each meal and snack to smooth out glucose response. It is also common for women with GDM to be advised to avoid fruits, fruit juices, bagels, and certain cereals at breakfast if the post breakfast blood sugar is out of range. Nutrition management is the only treatment necessary for up to 75% of women with GDM.

However, if nutritional management cannot keep blood glucose levels within goal range, insulin must be initiated. The goal of insulin therapy is to maintain glucose in the goal ranges. The regimen and doses should be tailored to address the specific times of day when the woman is experiencing higher glucose levels: either fasting or in the postmeal period. Human insulin should be used.

There is some recent data suggesting that some women with gestational diabetes may be managed with sulfonylurea oral hypoglycemic agents. The studies do not include a large enough number of women to ensure complete safety. However, many physicians are offering this as an option. In general, however, oral hypoglycemic agents are not recommended in pregnancy unless the benefits appear to clearly outweigh the risks.

Look to the Future

Women who have had GDM are at higher risk to also develop this condition in later pregnancies and should be screened early in any subsequent pregnancies. They are also at higher risk for Type 2 diabetes. Physical activity is a powerful force in reducing insulin resistance. Therefore, maintaining an active lifestyle, together with reasonable food management to prevent or control overweight are wise lifelong practices for the woman who has had GDM. These behaviors will be valuable for the growing family as well, since the offspring of GDM pregnancies are at increased risk of weight gain, glucose intolerance and Type 2 diabetes in childhood and adolescence.

For More Information

Original text by Betty Brackenridge, MS, RD, CDE on July 14, 2000
Updated September 13, 2003 based on comments by Dr. Bill Jones.
Updated December 18, 2005

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Last Updated: Sun Dec 18 11:30:41 2005
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