Screening is recommended for all pregnant women due to the poor predictive value of historical factors, and the fact that 4% of pregnant women will develop glucose intolerance.

Glucose intolerance develops later in pregnancy, so screening at 24-28 weeks is suggested to increase detection rate while providing time for treatment if necessary.

How screening is done:

• A 50 gram glucose challenge test is given at 24-28 weeks. This simply involves that you drink a bottle of very sweet, flavored liquid.
• You wait one hour and have a blood sample drawn.
• Depending on the cutoff value that your doctor's office uses, either 130 or 140 is considered "high."
• If your blood sugar at one hour is less than the cutoff, gestational diabetes is not likely and no further testing will be done, unless other factors (excessive growth of the baby, excess amniotic fluid, or sugar in your urine) indicate repeat testing.
• If your one-hour blood sugar is more than 130-140 a second test will be scheduled. This is the "glucose tolerance test" (see below).
• The GTT is done by drawing blood with you fasting, and then 3 times again at 1, 2, and 3 hours after drinking the glucose solution, this time with 100 milligrams of glucose.
• If you have a history of stillbirth, very large infants, are very obese, or had gestational diabetes in a prior pregnancy, your screening should be done earlier than 24 weeks.
• Screening may be omitted in patients younger than 25, not obese, with no family history of diabetes, and not in an ethnic group which has a high incidence of diabetes (Hispanic, African, Native American, Asian, and/or Pacific Island ancestry).

Followup for screen - positive patients:

• If 50-gram screen is >190 mg/dl, a fasting glucose should be checked

• If fasting glucose is elevated, the patient should be considered to have gestational diabetes

• Otherwise, administer 3-hour oral glucose tolerance test

• "Positive" 3-hour glucose tolerance test: 2 or more abnormal values out of 4:*
  • Fasting: 95 mg/dl
  • 1-hour: 180 mg/dl
  • 2-hour: 155 mg/dl
  • 3-hour: 140 mg/dl

*"Carpenter" criteria. More stringent (O'Sullivan) criteria also used are 105, 190, 165, and 145 mg/dl respectively.

A positive GTT will be first treated by diet, and blood sugars followed by you at home. If diet is not sufficient to bring blood sugar under control, insulin or an oral medication may be advised.