Gestational Diabetes: Diagnosis

Women who are considered at risk for gestational diabetes are given a screening test called a 50 gram glucose challenge between the 24th and 28th weeks of pregnancy (those with two or more risk factors may be tested earlier). The glucose challenge is performed by giving 50 grams of a glucose drink and then drawing a blood sample one hour later and measuring the level of blood glucose present. A blood sugar level of less than 140 mg/dl (7.8 mmol/L) at one hour is considered normal. Women with a blood sugar level greater than 140 mg/dl (7.8 mmol/L) may have gestational diabetes, and require a follow up test called a 3-hour oral glucose tolerance test (OGTT).

The OGTT requires dietary preparation. For three days prior to the test, 150 grams of carbohydrates should be consumed daily. Then a fasting period of 10 to 14 hours before the test is required; only water is allowed during this time. Because of the fasting component, the OGTT is usually performed first thing in the morning in a doctor’s office or outpatient lab. An initial blood sample is drawn to measure fasting blood glucose levels, and then the patient is given 100 grams of a glucose drink (the same type as that given in the glucose challenge) to consume. After that, blood samples are drawn every hour for 3 hours and blood glucose levels are measured.

The American Diabetes Association sets the following guidelines for results from the OGTT:

Fasting Blood Glucose Level≥95 mg/dl (≥5.33 mmol/L)

1 Hour Blood Glucose Level≥180 mg/dl (≥10 mmol/L)

2 Hour Blood Glucose Level≥155 mg/dl (≥8.6 mmol/L)

3 Hour Blood Glucose Level≥140 mg/dl (≥7.8 mmol/L)

If two or more blood sugar levels are higher than the levels indicated, gestational diabetes is diagnosed.¹

Reviewed by Francine Kaufman, MD. 4/08