Screening for GDM usually happens between the 24th and 28th week of your pregnancy, but if you are considered high risk for GDM you may be screened earlier (see RISK FACTORS).

The initial test will be conducted between 24 & 28 weeks of pregnancy-the glucose screening test or glucose challenge test -GCT. This test will not diagnose whether you have GDM but instead identify if there is a potential problem. If you test positive, you will need to take an additional test called the glucose tolerance test (GTT).

The GCT will require you to drink a very sweet solution containing 50 grams of glucose within a 5-minute timeframe. An hour later a blood sample will be taken to see how your body processed the sugar. If your results were abnormal then you will be required to take the GTT.

The GTT involves fasting overnight and taking a blood sample before you eat breakfast, to test your fasting blood sugar level. Next you will drink another sweet solution but this time it will contain about a 100 grams of glucose, and your blood sugar levels will be tested after one, two, and three hours. You'll need to sit in the waiting room during the entire three-hour test. If you have GDM, two or more of your blood sugar levels will meet or exceed the established levels listed in the table below.

Diagnosis of GDM with 100g glucose load
Blood sugar                                mg/dL
Fasting                                       95
1-hour                                        180
2-hours                                     155
3-hours                                     140