Glucose screening is screening for gestational diabetes and is generally performed routinely at 24-28 weeks in pregnant women who do not have risk factors for diabetes. If a woman has risk factors for diabetes, including gestational age over 35 years, overweight before pregnancy, previous history of abnormal glucose tolerance, history of polycystic ovary syndrome, history of adverse pregnancy, family history of diabetes or history of abnormal fetal development during pregnancy, the risk of gestational diabetes may increase. Patients with such high-risk factors need to be screened for diabetes at an appropriately earlier week, possibly as early as 12-24 weeks of gestation, to rule out diabetes. Patients undergoing a glucose tolerance test should fast for at least 8 hours, have a normal diet, rest and exercise 3 days before the test, and not exercise or smoke during the test.