Introduction to Gestational Diabetes

Approximately 3 to 5 percent of all pregnant women in the United States are diagnosed as having gestational diabetes. These women and their families have many questions about this disorder. Some of the most frequently asked questions are: What is gestational diabetes and how did I get it? How does it differ from other kinds of diabetes? Will it hurt my baby? Will my baby have diabetes? What can I do to control gestational diabetes? Will I need a special diet? Will gestational diabetes change the way or the time my baby is delivered? Will I have diabetes in the future?

This article will address these and many other questions about diet, exercise, measurement of blood sugar levels, and general medical and obstetric care of women with gestational diabetes. It must be emphasized that these are general guidelines and only your health care professional(s) can tailor a program specific to your needs. You should feel free to discuss any concerns you have with your doctor or other health care provider, as no one knows more about you and the condition of your pregnancy.

What is gestational diabetes and what causes it?

Diabetes (actual name is diabetes mellitus) of any kind is a disorder that prevents the body from using food properly. Normally, the body gets its major source of energy from glucose, a simple sugar that comes from foods high in simple carbohydrates (e.g., table sugar or other sweeteners such as honey, molasses, jams, and jellies, soft drinks, and cookies), or from the breakdown of complex carbohydrates such as starches (e.g., bread, potatoes, and pasta). After sugars and starches are digested in the stomach, they enter the blood stream in the form of glucose* (figure 1). The glucose in the blood stream becomes a potential source of energy for the entire body, similar to the way in which gasoline in a service station pump is a potential source of energy for your car. But, just as someone must pump the gas into the car, the body requires some assistance to get glucose from the blood stream to the muscles and other tissues of the body. In the body, that assistance comes from a hormone called insulin. Insulin is manufactured by the pancreas, a gland that lies behind the stomach. Without insulin, glucose cannot get into the cells of the body where it is used as fuel. Instead, glucose accumulates in the blood to high levels and is excreted or "spilled" into the urine through the kidneys.

Insulin: The Key to Turning Food into Energy

* For the purpose of this article, the words sugar and glucose are used synonymously.

When the pancreas of a child or young adult produces little or no insulin we call this condition juvenile-onset diabetes or Type I diabetes (insulin-dependent). This is not the type of diabetes you have. Unlike women with Type I diabetes, women with gestational diabetes have plenty of insulin. In fact, they usually have more insulin in their blood than women who are not pregnant. However, the effect of their insulin is partially blocked by a variety of other hormones made in the placenta, a condition often called insulin resistance.

The placenta performs the task of supplying the growing fetus with nutrients and water from the mother's circulation. It also produces a variety of hormones vital to the preservation of the pregnancy. Ironically, several of these hormones such as estrogen, cortisol, and human placental lactogen (HPL) have a blocking effect on insulin, a "contra-insulin" effect. This contra-insulin effect usually begins about midway (20 to 24 weeks) through pregnancy. The larger the placenta grows, the more these hormones are produced, and the greater the insulin resistance becomes. In most women the pancreas is able to make additional insulin to overcome the insulin resistance. When the pancreas makes all the insulin it can and there still isn't enough to overcome the effect of the placenta's hormones, gestational diabetes results. If we could somehow remove all the placenta's hormones from the mother's blood, the condition would be remedied. This, in fact, usually happens following delivery.

How does gestational diabetes differ
from other types of diabetes?

There are several different types of diabetes. Gestational diabetes begins during pregnancy and disappears following delivery. Another type is referred to as juvenile-onset diabetes (in children) or Type I (in young adults). These individuals usually develop their disease before age 20. People with Type I diabetes must take insulin by injection every day. Approximately 10 percent of all people with diabetes have Type I (also called insulin-dependent diabetes).

Type II diabetes or noninsulin-dependent diabetes (formerly called adult-onset diabetes) is also characterized by high blood sugar levels, but these patients are often obese and usually lack the classic symptoms (fatigue, thirst, frequent urination, and sudden weight loss) associated with Type I diabetes. Many of these individuals can control their blood sugar levels by following a careful diet and exercise program, by losing excess weight, or by taking oral medication. Some, but not all, need insulin. People with Type II diabetes account for roughly 90 percent of all diabetics.

Who is at risk for developing gestational diabetes
and how is it detected?

Any woman might develop gestational diabetes during pregnancy. Some of the factors associated with women who have an increased risk are obesity; a family history of diabetes; having given birth previously to a very large infant, a stillbirth, or a child with a birth defect; or having too much amniotic fluid (polyhydramnios). Also, women who are older than 25 are at greater risk than younger individuals. Although a history of sugar in the urine is often included in the list of risk factors, this is not a reliable indicator of who will develop diabetes during pregnancy. Some pregnant women with perfectly normal blood sugar levels will occasionally have sugar detected in their urine.

The Council on Diabetes in Pregnancy of the American Diabetes Association strongly recommends that all pregnant women be screened for gestational diabetes. Several methods of screening exist. The most common is the 50-gram glucose screening test. No special preparation is necessary for this test, and there is no need to fast before the test. The test is performed by giving 50 grams of a glucose drink and then measuring the blood sugar level l-hour later. A woman with a blood sugar level of less than 140 milligrams per deciliter (mg/dl) at l-hour is presumed not to have gestational diabetes and requires no further testing. If the blood sugar level is greater than 140 mg/dl the test is considered abnormal or "positive." Not all women with a positive screening test have diabetes. Consequently, a 3-hour glucose tolerance test must be performed to establish the diagnosis of gestational diabetes.

If your physician determines that you should take the complete 3-hour glucose tolerance test, you will be asked to follow some special instructions in preparation for the test. For 3 days before the test, eat a diet that contains at least 150 grams of carbohydrates each day. This can be accomplished by including one cup of pasta, two servings of fruit, four slices of bread, and three glasses of milk every day. For 10 to 14 hours before the test you should not eat and not drink anything but water. The test is usually done in the morning in your physician's office or in a laboratory. First, a blood sample will be drawn to measure your fasting blood sugar level. Then you will be asked to drink a full bottle of a glucose drink (100 grams). This glucose drink is extremely sweet and occasionally makes some people feel nauseated. Finally, blood samples will be drawn every hour for 3 hours after the glucose drink has been consumed. The normal values for this test are shown in table 1.

TABLE 1.
Three Hour Glucose Tolerance Test for Gestational Diabetes

If two or more of your blood sugar levels are higher than the diagnostic criteria, you have gestational diabetes. This testing is usually performed at the end of the second or the beginning of the third trimester (between the 24th and 28th weeks of pregnancy) when insulin resistance usually begins. If you had gestational diabetes in a previous pregnancy or there is some reason why your physician is unusually concerned about your risk of developing gestational diabetes, you may be asked to take the 50-gram glucose screening test as early as the first trimester (before the 13th week). Remember, merely having sugar in your urine or even having an abnormal blood sugar on the 50-gram glucose screening test does not necessarily mean you have gestational diabetes. The 3-hour glucose tolerance test must be abnormal before the diagnosis is made.

How does gestational diabetes affect pregnancy and will it hurt my baby?

The complications of gestational diabetes are manageable and preventable. The key to prevention is careful control of blood sugar levels just as soon as the diagnosis of gestational diabetes is made.

You should be reassured that there are certain things gestational diabetes does not usually cause. Unlike Type I diabetes, gestational diabetes generally does not cause birth defects. For the most part, birth defects originate sometime during the first trimester (before the 13th week) of pregnancy. The insulin resistance from the contra-insulin hormones produced by the placenta does not usually occur until approximately the 24th week. Therefore, women with gestational diabetes generally have normal blood sugar levels during the critical first trimester.

*O'Sullivan, J. B. Establishing Criteria for Gestational Diabetes. Diabetes Care 3: 437-439, 1980.

One of the major problems a woman with gestational diabetes faces is a condition the baby may develop called macrosomia. Macrosomia means "large body" and refers to a baby that is considerably larger than normal. All of the nutrients the fetus receives come directly from the mother's blood (figure 2). If the maternal blood has too much glucose, the pancreas of the fetus senses the high glucose levels and produces more insulin in an attempt to use the glucose. The fetus converts the extra glucose to fat. Even when the mother has gestational diabetes, the fetus is able to produce all the insulin it needs. The combination of high blood glucose levels from the mother and high insulin levels in the fetus results in large deposits of fat which causes the fetus to grow excessively large, a condition known as macrosomia. Occasionally, the baby grows too large to be delivered through the vagina and a cesarean delivery becomes necessary. The obstetrician can often determine if the fetus is macrosomic by doing a physical examination. However, in many cases a special test called an ultrasound is used to measure the size of the fetus. This and other special tests will be discussed later.

In addition to macrosomia, gestational diabetes increases the risk of hypoglycemia (low blood sugar) in the baby immediately after delivery. This problem occurs if the mother's blood sugar levels have been consistently high causing the fetus to have a high level of insulin in its circulation. After delivery the baby continues to have a high insulin level, but it no longer has the high level of sugar from its mother, resulting in the newborn's blood sugar level becoming very low. Your baby's blood sugar level will be checked in the newborn nursery and if the level is too low, it may be necessary to give the baby glucose intravenously. Infants of mothers with gestational diabetes are also vulnerable to several other chemical imbalances such as low serum calcium and low serum magnesium levels.

All of these are manageable and preventable problems. The key to prevention is careful control of blood sugar levels in the mother just as soon as the diagnosis of gestational diabetes is made. By maintaining normal blood sugar levels, it is less likely that a fetus will develop macrosomia, hypoglycemia, or other chemical abnormalities.

What can be done to reduce problems associated
with gestational diabetes?

In addition to your obstetrician, there are other health professionals who specialize in the management of diabetes during pregnancy including internists or diabetologists, registered dietitians, qualified nutritionists, and diabetes educators. Your doctor may recommend that you see one or more of these specialists during your pregnancy. In addition, a neonatologist (a doctor who specializes in the care of newborn infants) should also be called in to manage any complications the baby might develop after delivery.

One of the essential components in the care of a woman with gestational diabetes is a diet specifically tailored to provide adequate nutrition to meet the needs of the mother and the growing fetus. At the same time the diet has to be planned in such a way as to keep blood glucose levels in the normal range (60 to 120 mg/dl). Specific details about diet during pregnancy are discussed later.

An obstetrician, diabetes educator, or other health care practitioner can teach you how to measure your own blood glucose levels at home to see if levels remain in an acceptable range on the prescribed diet. The ability of patients to determine their own blood sugar levels with easy-to-use equipment represents a major milestone in the management of diabetes, especially during pregnancy. The technique called "self blood glucose monitoring" (discussed in detail later) allows you to check your blood sugar levels at home or at work without costly and time-consuming visits to your doctor. The values of your blood sugar levels also determine if you need to begin insulin therapy sometime during pregnancy. Short of frequent trips to a laboratory, this is the only way to see if blood glucose levels remain under good control.

NIDDK