During pregnancy, it is standard to screen women for gestational diabetes around the beginning of the third trimester. Nearly all pregnant women will experience a rise in glucose intolerance due to hormonal changes; this rise is usually minimal in the early stages of pregnancy. However, as pregnancy progresses and hormonal changes become more aggressive, expecting moms are at a higher risk of developing gestational diabetes. 
But why are those pesky hormones causing the problem in the first place? Hormones produced in the placenta are the driving force behind transferring nutrients from mom to baby; these hormones also prevent mom from developing low blood sugar. This directly correlates to an increase in glucose intolerance. To combat this, the body makes more insulin. Unfortunately, sometimes the body is unable to produce enough insulin and expecting moms wind up with gestational diabetes. 
Testing for Gestational Diabetes
Due to the complications and risks of gestational diabetes (fetal defects, miscarriage, emergency caesareans, etc), expecting moms are tested around 24-28 weeks gestation. Some individuals in particular are at a higher risk of developing gestational diabetes. They include individuals who are overweight, certain ethnic groups (Hispanic, black, Asian, and Native American), individuals with a family history of diabetes, and more. 
The week prior to the 2 hour glucose test, individuals are asked to eat as they normally would. Then, at least 8 hours prior to the screening, individuals are required to fast. When the patient arrives for the two hour glucose test, their blood is drawn for a baseline glucose level. Patients are then asked to drink 75g of a glucose solution and one hour later their blood is drawn. Their blood is drawn a second time another hour later (two hours total, hence the two hour glucose test). 
During this time, individuals are not permitted to eat or drink anything other than the glucose solution they are given prior to testing. Some individuals may experience nausea, fainting, and shortness of breath during testing. Should the patient vomit, the test is void and will need to be rescheduled.
Why the 2 Hour Glucose Test?
This screening is relatively new, but has positive implications over its predecessor. The original method for gestational diabetes screening only tested a patient’s blood once after ingesting the glucose solution. If blood sugar levels were higher than normal, then the patient’s blood would be drawn every hour for three hours. However, if the first draw came back in normal ranges, a patient could go undiagnosed.
Furthermore, the range for normal glucose levels during pregnancy has shrunk. One study found “Blood sugar levels that were once considered in the normal range are now seen as causing a sharp increase in the occurrence of overweight babies with high insulin levels, early deliveries, cesarean section deliveries and potentially life-threatening preeclampsia.”  The two hour glucose test can catch cases of mild gestational diabetes, which allows pregnant women to make the necessary changes to their diet and life style as well as monitor their blood sugar levels.