- Carla Janzen, MD, Jeffrey S. Greenspoon, MD
Essentials of Diagnosis
- Any degree of glucose intolerance with onset or first recognition during pregnancy.
- In the majority of cases of GDM, glucose regulation will return to normal after delivery.
The above definition applies regardless of whether insulin or only diet modification is used for treatment or whether the condition continues after pregnancy. It is possible that unrecognized glucose intolerance may have antedated or begun concomitantly with pregnancy.
Gestational diabetes may be screened for by drawing a 1-hour glucose level following a 50-gram glucose load, but is definitively diagnosed only by an abnormal 3-hour GTT following a 100-gram glucose load. Such persons are not within the norm (95%) for pregnancy.
The growth and maturation of the fetus are closely associated with the delivery of maternal nutrients, particularly glucose. This is most crucial in the third trimester and is directly related to the duration and degree of maternal glucose elevation. Thus the negative impact is as highly diverse as the variety of carbohydrate intolerance that women bring to pregnancy. In those with severe abnormalities, there is an increased rate of miscarriage, congenital malformations, prematurity, pyelonephritis, preeclampsia, in utero meconium, fetal distress, cesarean section deliveries, and stillbirth.
Incidence & Etiology
Inability to maintain glucose levels required by the body for proper functioning is a growing health problem in the United States; thus it is not surprising that more women are found during pregnancy to be unable to attain the low glucose levels required for proper fetal growth. The incidence of gestational diabetes varies from 12% in racially heterogeneous urban regions to 1% in rural areas with a predominantly white population.
Gestational diabetes is pathophysiologically similar to type 2 diabetes. Approximately 90% of the persons identified have a deficiency of insulin receptors (prior to pregnancy) or a marked increase in weight in the abdominal region. The other 10% have deficient insulin production and will proceed to develop mature-onset insulin-dependent diabetes.
Similarly to women with type 2 diabetes, the women most likely to develop gestational diabetes are those who are overweight, with a body habitus often described as "apple shaped." HPL blocks insulin receptors and increases in direct linear relation to the length of pregnancy. Insulin release is enhanced in an attempt to maintain glucose homeostasis. The patient experiences increased hunger due to the excess insulin release as a result of elevated glucose levels. This insulin release further decreases insulin receptors due to elevated hormonal levels. Thus the vicious cycle of excess appetite with weight gain occurs. Few other symptoms mark this condition.
Glucosuria is a common finding in pregnancy due to increased glomerular filtration and is therefore unreliable as a means of diagnosis. Glucose screening should be done in every pregnant patient at or no later than 28 weeks' gestation, since risk factors are insufficient to identify all women with gestational diabetes. Ultrasound findings of fetal weight ≥ 70% for gestational age, polyhydramnios (AFI ≥ 20), midline congenital anomalies, or an abdominal circumference measurement that exceeds the femur growth by 2 weeks merit an immediate 3-hour GTT. Other clinical findings indicating possible diabetes are edema developing early in pregnancy and excessive weight gain.
Initial screening is accomplished by ingestion of 50 grams of glucose (usually chilled glucola) at any time of the day and without regard to prior meal ingestion. The sensitivity and specificity are based on the cutoff value used to indicate a positive result (Table 18-4); however, screening is not as reproducible from day to day as would be desired. If screening is positive, the patient is advised to follow a carbohydrate loading diet for 3 days and then have a full 3-hour glucose tolerance test. A simple carbohydrate loading diet is all the pasta and starches she can eat at each meal and one candy bar per day. For the GTT, the patient fasts, then receives 100 grams of glucose after a fasting glucose level is obtained. A blood sample is then taken every hour for 3 hours. The patient is advised to sit quietly during the test to minimize the impact of exercise on glucose levels.
The glucose values initially used to detect gestational diabetes were determined by O'Sullivan and Mahan in a retrospective study designed to detect risk of developing type 2 diabetes in the future. The values were set using whole blood and required two values reaching or exceeding the value to be positive. Subsequent information has led to alteration in O'Sullivan's criteria. For example, there is growing evidence that one value is sufficient to make an impact on the health of the fetus, and is now the criterion used by most clinicians to initiate treatment. Whole blood glucose values are lower than plasma levels due to glucose uptake by hemoglobin. The present values used by the American College of Obstetricians and Gynecologists are based on a theoretical increase in hemoglobin and plasma with pregnancy. Recently, a study using all three methods of glucose determination on the same samples have disproved the theoretical values and are listed in Table 18-5.
The key to therapy in most patients is diet and exercise (because of the paucity of insulin receptors). This makes therapy more difficult than with the insulin-deficient patient, in whom exogenous insulin may be easily administered. Therapy in the type 2 diabetic is based on the patient's motivation and ability to change lifestyle. Exercise of the non-weight-bearing type (noted previously) is encouraged as even short exercise periods have a major benefit.
Every care provider must stress the importance of diet. Soluble fiber is invaluable to provide satiety and improve insulin receptor numbers and sensitivity. Carbohydrate restriction has been shown to improve glycemic control in diet-controlled GDM. Fats must be reduced because of their negative impact on insulin receptors. Calories should be prescribed at 20-25/kcal per kilogram of present body weight (generally 1800-2400 kcal). Massively obese patients have a reduction in their metabolism rate; therefore, it is better to start low and increase the calories as needed. Food records are kept for 1 week, and the content and calories are reviewed by the dietitian with helpful suggestions on improving favorite dishes to be included in the diet. The patient is instructed to particularly note all food taken in when a 1-hour postprandial glucose value is 130 mg/dL or greater. The memory reflectance glucose meters are invaluable in assisting the patient to learn the proper diet and the impact of her actions on glucose levels. Insulin is added as needed for glucose control only after clear dietary errors are noted and attempts at correction are done. Approaches to initiating insulin therapy vary, but should remain as simple as possible.
A minimum of 2 visits to the dietitian encourages education and interaction over dietary questions. The customization of diet to ethnic foods is often invaluable in obtaining dietary compliance. The encouragement of other family members to participate in dietary counseling assists their support for the patient and is key to making familial dietary changes. The patient often benefits from direct contact with the dietitian when glucose levels are erratic, when her weight fails to meet expected guidelines, when she is having difficulty with calorie counting, or when she increases daily calories more than 300 kcal over guidelines.
The patient checks her glucose 4 times daily (eg, fasting, and 1-hour postprandial breakfast, lunch, dinner). The desired values are a fasting level of 70-90 mg/dL and a 1-hour level of 130 mg/dL. The average glucose levels should be 90 mg/dL.