What is gestational diabetes?

Causes and Symptoms

Gestational diabetes mellitus (GDM) is the medical term describing a type of diabetes mellitus that is first diagnosed during a woman’s pregnancy. Diabetes is a condition in which blood glucose is not kept within a normal range. Normally, insulin is a key regulator of blood glucose. In diabetes, insulin may be absent, be present in insufficient amounts, or be ineffective. Gestational diabetes occurs in 1 to 14 percent of all pregnancies. There is variance in incidence rates as a result of race or ethnicity, advanced age, and genetic predisposition. In general, women with a family history of diabetes are more likely to develop gestational diabetes, as are women who were overweight or obese prior to pregnancy, are carrying multiple fetuses, or have previously had unexplained miscarriage or stillbirth. Women with gestational diabetes often experience no symptoms themselves; when symptoms do present in the mother, they may include increased hunger, thirst, and urination, along with weight loss and fatigue.

Gestational diabetes generally develops midway through pregnancy. Testing is typically scheduled for between twenty-four and twenty-eight weeks into the pregnancy. A two-step approach is used to screen women who are not at high risk for diabetes. The first step is the 50-gram oral glucose tolerance test. For this test, the woman is given 50 grams of glucose in solution after having fasted overnight. Her blood glucose is tested one hour after drinking the solution. If her blood glucose is above a normal range, the next step is a 100-gram three-hour oral glucose tolerance test. Normally, a person’s insulin would react to the ingested glucose to keep the blood glucose within a normal range. If that does not occur, and blood glucose remains high, then a diagnosis of gestational diabetes is made.

As maternal blood glucose rises, so does the risk of fetal complications. The most common complication is fetal macrosomia
, or having a birth weight greater than or equal to 4.5 kilograms. Fetal macrosomia is associated with an increased risk of birth trauma, especially shoulder dystocia. Infants this large may require a cesarean section for birth, which itself has greater health risks than a vaginal birth. Additionally, higher maternal glucose levels lead to poorer placental functioning at an earlier point in pregnancy. While the placenta is designed to work as a filtering mechanism for thirty-eight to forty-two weeks, in gestational diabetics, the placenta often begins to malfunction by thirty-seven weeks. Therefore, infants of diabetic mothers are delivered early (at thirty-seven weeks) to avoid placental malfunction. Pre-eclampsia, or high blood pressure, in the mother can also develop and may lead to preterm birth.

When maternal blood glucose levels are elevated above normal, the fetus is stimulated to increase production of insulin. Although this manages the problem of the increased blood glucose for the fetus, it also has negative consequences. If the mother’s blood glucose has been elevated just prior to delivery, then the infant’s insulin level will be elevated. After delivery, the infant’s insulin level may remain elevated, although there is no longer a need for it. This can lead to hypoglycemia, or low blood glucose. Continued hypoglycemia can lead to coma or death for the newborn. In addition, high insulin levels and poor control of the mother’s blood glucose is associated with problems with the infant’s heart and lung function. Other potential complications include respiratory distress and jaundice (yellowing of the skin).

Treatment and Therapy

Diet is the primary treatment for gestational diabetes. Depending on the meal planning approach, a certain number of calories and/or a certain amount of carbohydrates is prescribed. The total carbohydrates to be eaten is about 40 to 45 percent of total daily calories. Calories should be prescribed to allow for recommended weight gain during pregnancy and to prevent blood glucose from being either too high or too low. If caloric intake is too high, then the blood glucose level will rise, which is detrimental to the fetus. If caloric intake is too low, then the body will break down the mother’s body fat or protein reserves to supply the needed energy. When this occurs, breakdown products called "ketones" are produced. Ketones in the mother’s blood are also detrimental to the fetus.

Consistency, in the form of eating the same amount of food at the same time each day, is important. This is most likely to occur if the individual eats small, frequent meals throughout the day. The diet must support three outcomes: blood glucose levels within a target range, adequate nutrient intake to support the pregnancy, and appropriate weight gain for pregnancy. If these three outcomes cannot be achieved by diet alone, then insulin will be used to achieve the desired blood glucose level. Oral hypoglycemic medications such as sulfonylureas, either alone or in combination with insulin, may be helpful in blood glucose regulation; patients should discuss the potential risks and benefits of these medications with their physicians.

Because achieving a normal or near-normal blood glucose level is so critical, the woman will monitor her blood glucose at home using a fingerstick blood sample and a home glucometer. Blood glucose levels are usually tested three to four times a day, although some women will need to test their blood glucose six times each day. Decisions about adjustments in diet and insulin will be based on blood glucose levels.

Regular exercise is also recommended as a means of regulating blood glucose levels. Patients should consult with their physicians about what types of exercise are safe and appropriate during the various stages of pregnancy.

Usually blood glucose levels normalize postpartum, and continued diet or medication therapy is not needed. However, women who develop gestational diabetes have a higher likelihood of developing type 2 diabetes mellitus later in life. For women who have developed gestational diabetes, an oral glucose tolerance test is administered six to eight weeks postpartum and then at three-year intervals. These women should maintain an optimal weight, since obesity is strongly associated with onset of type 2 diabetes.

Perspective and Prospects

Observations in the 1950s and 1960s that infants born to mothers who had an elevated blood glucose level had a higher rate of morbidity and mortality led to the screening, diagnosis, and treatment guidelines used today. Adherence to these guidelines has greatly improved the health of infants born to mothers with gestational diabetes. However, these infants do have a greater risk of becoming obese and/or developing diabetes in adolescence. Additionally, daughters of mothers who have had gestational diabetes have a greater likelihood of developing gestational diabetes themselves.

Bibliography

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