The Glucose Challenge Test (GCT) is used to screen expectant mothers for gestational diabetes. Because there are no signs or symptoms of gestational diabetes, the GTT has become standard for every pregnant woman. Your glucose levels will also be monitored using the urine dipstick at every midwife visit. The GCT will be done between 26 and 28 weeks gestation (28 and 30 weeks pregnancy).
Fasting glucose does not do a good job of differentiating women with gestational diabetes from those who do not have gestational diabetes. Because of that, women are asked to fast and then ingest a high glucose solution to see how their body handles the sugar. There is disagreement on the necessity of this test, and the importance of the results.
For example, one of the biggest risks for gestational diabetes is macrosomia (a very large baby), however most macrosomic babies are born to women who have normal glucose challenge tests. Another concern is that the test is only measuring what is normal in pregnancy, that expectant mother's body is designed to keep the blood sugar level higher for longer to allow the baby to get adequate nutrition.
What you need to know
The level of error for predicting glucose problems decreases as the acceptable level of glucose is increased. When the standard cut off of 7.8 mmol/l is used, 45.8% of women were found to actually have gestational diabetes. When the cut off was raised to 12 mmol, over 95% of women tested actually had gestational diabetes.
If you do test as positive for gestational diabetes you will have best results if you make changes to your diet and exercise and have your blood sugar levels tested regularly. If you are not testing your blood sugar, you have no way of ensuring what you are doing is producing the desired effect.
You will be asked to fast for a specified amount of time before the test. To start the test, your blood will be drawn to get a fasting blood glucose level. You will then drink a high-glucose liquid and have your blood drawn again in one hour. If your results are high on the one hour GCT, you will be asked to return to have the Glucose Tolerance Test, which is a similar test with a three hour wait time.
Wong, V.W., Garden, F., and Jalaludin, B. (2009) Hyperglycaemia following glucose challenge test during pregnancy: When can a screening test become diagnostic? Diabetes Research and Clinical Practice 83:394-396.