Gestational diabetes mellitus (GDM) is diabetes that is first recognized in the second or third trimester of pregnancy, in someone who did not clearly have diabetes before pregnancy. Its diagnostic thresholds are lower than those used for other types of diabetes mellitus, and the reason is the target of the test: the values were chosen from the glucose levels at which the risk of adverse pregnancy outcomes — such as a large baby, preeclampsia (a hypertensive disorder of pregnancy) and birth complications — starts to rise, rather than from the mother’s later risk of diabetes.
The one-step 75-gram OGTT
The test recommended for the diagnosis of gestational diabetes mellitus is the 2-hour 75-gram oral glucose tolerance test (OGTT), in which plasma glucose is measured while fasting and again at set times after a glucose drink. It is performed at 24-28 weeks of gestation, in the morning after an overnight fast of at least 8 hours. The diagnosis is made when any one of the following plasma glucose values is met or exceeded:
- fasting plasma glucose → 92 mg/dl (5.1 mmol/L)
- after one hour → 180 mg/dl (10.0 mmol/L)
- after two hours → 153 mg/dl (8.5 mmol/L)
A single abnormal value is enough here, because the one-step test was built around the glucose level at which pregnancy risk begins to rise, in contrast to the non-pregnant criteria, where confirmation is needed.
The two-step test
The alternative is a two-step pathway, which is also accepted and is more common in the United States; it is named in contrast to the one-step test above, which makes the diagnosis in a single sitting. The first step is a screening test: 50 grams of glucose are given regardless of fasting state, and if the plasma glucose one hour later reaches 130 mg/dl (7.2 mmol/L) — some protocols use 135 or 140 mg/dl — the test is positive and the confirmation test must be performed. The second step is a 100-gram OGTT after a night of fasting, and the diagnosis of GDM is confirmed if at least two of the following values are met or exceeded:
- fasting plasma glucose → ≥ 105 mg/dl (5.8 mmol/L)
- one hour plasma glucose → ≥ 190 mg/dl (10.5 mmol/L)
- two hours plasma glucose → ≥ 165 mg/dl (9.2 mmol/L)
- three hours plasma glucose → ≥ 145 mg/dl (8.0 mmol/L)
These are the higher National Diabetes Data Group thresholds. The American Diabetes Association (ADA) now prefers the lower Carpenter-Coustan thresholds for the 100-gram OGTT — fasting ≥ 95 mg/dl (5.3 mmol/L), one hour ≥ 180 mg/dl (10.0 mmol/L), two hours ≥ 155 mg/dl (8.6 mmol/L), three hours ≥ 140 mg/dl (7.8 mmol/L).