Diabetes in Pregnancy - 5 - Diagnosis of Gestational Diabetes
Routine screening of all patients (except of course those who are already diabetic) for GDM has become a standard of care. GDM is associated with large (macrosomic) babies, risk of trauma during delivery (for both mothers and babies), neonatal hypoglycemia (secondary to hyperinsulinemia), hypocalcemia, hyperbilirubinemia, respiratory complications if delivered prior to term, and need for cesarean delivery. Poorly-controlled GDM also places the baby at risk for intrauterine fetal demise. Although over the years, some have argued that it is not necessary to screen all pregnant women, that has proven to be a battle not worth fighting. Screening is very simple and inexpensive to perform. If women were only screened based on ‘risk factors’ alone, about 50% of GDM would be missed (Jovanovic, et al., Diabetes 1985;34:21). If women who were not obese were excluded from screening, 10% of GDM would not be diagnosed and 16% of these women would have significant blood glucose levels that would require insulin therapy (Moses, et al., Diabetes care 1998;21:1803). Absence of diabetes with an earlier pregnancy is also inadequate to exclude GDM in a subsequent pregnancy, especially if a woman is significantly older or has gained weight since the previous pregnancy.
Different approaches to screening are employed around the world. In the U.S., the most widely accepted approach is to screen women at 24-28 weeks gestation who have not been shown or suspected to have hyperglycemia prior to this time. The glucose challenge test (GCT) is done by giving women a 50g oral glucose load, without regard to time of day or when they have last eaten, and then checking a plasma glucose level 60 minutes later. Although different institutions use different cutoff values to identify women for further testing, most use values > 140 mg/dL as an indication to perform a full 3 hour glucose tolerance test (GTT). Using this cutoff at this point in pregnancy will result in the need for a GTT in about 15% of women, but will detect 85-90% of gestational diabetics. Using a cutoff of 130 mg/dL for the GCT will pick up nearly 100% of GDM, but will be at the cost of having to perform a GTT on 25% or more of women.
The GTT is performed as follows: The patient is advised to eat her usual diet for at least 3 days prior to the test and to maintain her normal level of physical activity. Some suggest a diet that includes at least 150 g/day of carbohydrate. Then, after an overnight fast of at least 8 hours (but less than 14 hours), a fasting plasma glucose level is obtained and the woman is given a 100g oral glucose load. She is asked to limit her activity and not smoke during the test. Plasma glucose levels are then obtained 1 hour, 2 hours, and 3 hours later. Using either the recommendations of the National Institutes of Health Diabetes Data Group (NDDG) (Diabetes 1979;28:1039) or the values established by Carpenter and Coustan (Am J Obstet Gynecol 1982;144:768-73) shown in parentheses, abnormal values for each of the plasma glucose time points are as follows: fasting 105 (95); 1 hour 190 (180); 2 hour 165 (155); and 3 hour 145 (140). If a woman has two or more of these values abnormal, she is considered to have gestational diabetes. The attendant morbidity during pregnancy has been shown to be comparable regardless of the criteria set used. In all, 15-20% of women who end up requiring a GTT based on the initial GCT will be found to have GDM.
There are some caveats to screening that are probably worth mentioning although standard approaches under the circumstances described have not been universally adopted. Although routine screening is usually done at 24-28 weeks, women with significant risk factors can be screened earlier in pregnancy. These include women who are obese, previously had GDM or a macrosomic baby, previously lost a baby in utero (particularly if the baby was large for gestational age) or had a delivery complicated by shoulder dystocia or a baby who had neonatal hypoglycemia, have a strong family history of diabetes, glucose detected in their urine or an unexpectedly high blood glucose level at the time of a random determination. Women who pass their initial GCT at 24-28 can also be rescreened later in pregnancy if they have any of these risk factors or if they develop a large baby, excessive amniotic fluid (polyhydramnios), or excessive weight gain during their pregnancies.
Women who have a very high blood glucose level (> 190 mg/dL) on their 1 hour GCT, have a 90% chance of being diabetic. Under these circumstances, it is prudent to check the fasting blood glucose level prior to proceeding with the full 3 hour GTT, and if this is > 95 mg/dL, declare them to have GDM and treat accordingly. Women who are found to have one elevated value at the time of their 3 hour GTT, should probably have the GTT (not the GCT) repeated at 32-34 weeks because 10-15% of GDM will not develop until later in pregnancy. An alternative to screening in women who have previously had GDM and are well-motivated is to counsel them regarding diet and exercise early in pregnancy and, if they are familiar with the use of a glucometer for self-glucose testing from the prior pregnancy, to have them begin checking fasting and 2 hour postprandial (after eating) blood sugars periodically. (Specifics regarding self-testing will be discussed further in our next post). The advantage of this approach is that it allows early intervention when their blood sugar control begins to deteriorate during the pregnancy.
Once the diagnosis of GDM has been established, a management program should be implemented and that will be the subject of our next post in this series….