Oral Glucose Tolerance Test. This test is usually completed between 24-28 weeks of pregnancy. For this test you will need to fast for between 8-12 hours before. You may also be asked not to take certain medications during this period. During the test you will initially have a blood test, then drink a sugary drink, wait for approximately two hours, then have another blood test. These two results are then used to diagnose gestational diabetes or confirm the absence of it.