The routine test used to diagnose gestational diabetes is called a glucose challenge. The woman consumes a drink with 50 grams of glucose; an hour later, her blood is tested to see how well the body has processed that glucose. If her blood glucose level is 140 milligrams/deciliter or more, the doctor will prescribe a second test, known as a glucose tolerance test, to confirm the diagnosis.

After a fast of eight to 12 hours, the woman drinks a liquid with 100 grams of glucose. Then her blood is tested four times, once at baseline and then again each hour for three hours. Two abnormal readings indicate gestational diabetes; one suggests the woman should be screened again in a month.

Pregnant women at very low risk for gestational diabetes--that is, those who are a member of an ethnic group that has a low prevalence of diabetes, are younger than 25, of normal weight before becoming pregnant, with no close family history of diabetes, and no history of abnormal glucose tolerance or troublesome pregnancies--might not be screened at all. Women who are neither at high or low risk should be screened between 24 and 28 weeks into pregnancy. Women at high risk might be screened earlier, although it is not always possible to detect gestational diabetes much before 24 weeks.