What is Gestational Diabetes?

The placenta (or afterbirth) is the organ between you and your baby that is responsible for delivering nutrients and oxygen to your baby. Babies use mostly sugar for food so the placenta tries to make as much sugar (glucose) available to the baby by making sugar unavailable to you. The placenta does this by producing hormones that make your body's insulin ineffective. Because insulin is required for sugar to enter your cells, the sugar rises in your blood. As the placenta becomes larger it produces more hormones to counteract the effect of insulin. The end result is that most pregnant women become mildly intolerant to extra sugar in their diet. However, some women who are already predisposed towards diabetes become overtly diabetic. Diabetes diagnosed during pregnancy is called gestational diabetes (GDM).

Testing for Diabetes

Your doctor may test you if he thinks you are at risk for developing GDM. You will be asked to drink a syrup like beverage. After an hour your blood is tested for excessive sugar. If the sugar level is too high ( > 140 mg/dl ) you will need to take an additional three-hour test. From 3% to 12% of all pregnancies are diagnosed with diabetes. 

Consequences of Diabetes

Women who are diabetic even when they are not pregnant are called pregestational diabetics. In pregestational diabetics fasting blood sugars persistently greater than 120 mg/dl in early pregnancy can cause miscarriage and birth defects. Mothers who are diabetic only when they are pregnant (GDM) do not have higher rates of birth defects or miscarriage. 

Later in pregnancy the excessive sugar in either type of diabetic crosses the placenta to the baby. The mother's insulin does not cross the placenta so the baby produces extra insulin to get rid of the overabundance of sugar. The consequences are the baby grows, and grows, and grows. Eventually some babies become "macrosomic" (big body) and may have difficulty delivering naturally. In addition, because the baby is producing insulin at such a high rate a sudden interruption (such as birth) of its sugar supply may cause the baby's blood sugar to drop dangerously low at birth ("insulin shock"). Moreover, excessive blood sugar has been shown to decrease the transfer of oxygen to the fetus in experimental studies. The latter effect may account for the higher rate of stillbirths seen in poorly controlled diabetic pregnancies. 

Lastly uncontrolled diabetes places the mother at risk for developing polyhydramnios (excessive amniotic fluid) and pre-eclampsia (toxemia).

Treatment

The first step in treatment is usually an American Diabetes Association (ADA) diet composed of :

• Carbohydrate 35-45% 
• Protein 20-25% 
• Fat 35-40%

In addition a minimum of three episodes of exercise per week is also recommended. The sugar lowering effect of exercise may not be seen for 2 to 4 weeks. 

If diet and exercise don't keep blood sugar controlled, then your doctor will likely prescribe insulin. Remember insulin doesn't cross the placenta so it has no direct effect on your baby. Although, the thought of daily injections is very unpleasant for most patients oral medications have yet to be proven as effective as insulin in controlling blood sugar during pregnancy. The major side effect of insulin therapy is possible low blood sugar (hypoglycemia). Hypoglycemia appears to affect the mother more than the fetus. 

The Risk of Developing Diabetes Later in Life

Approximately 15% to 20% of GDM patients require insulin therapy. Whether a patient subsequently develops diabetes seems to depend on their fasting glucose level. If fasting glucose levels during pregnancy are 105 to 130 mg/dl, 50% of mothers will subsequently be found to be overtly diabetic. If fasting blood sugar is > 130 mg/dL then 86 % of women will become overtly diabetic. For the vast majority of women diagnosed with GDM once the placenta has delivered blood sugars return to normal. However, the only way to resolve whether a patient is truly diabetic is to test them six weeks after delivery. It is important that this follow up be done, because the deleterious effects of diabetes on the mother's health and her subsequent pregnancies are easier to prevent than to treat. 

SEE ALSO: DIABETES IN PREGNANCY
 

Created: 12/18/2002
Update: 11/2/2004