Most women are screened for gestational diabetes during their pregnancy. If a woman is under age 25 and has no risk factors for the disorder, she may not be tested for the condition. Otherwise, women are screened at different times during the pregnancy based on their risk level.

A woman is considered high risk if she is overweight or obese, has had gestational diabetes before, or has a family history of diabetes. These women may be checked for gestational diabetes at the first prenatal visit. If the results are normal, the women are usually checked again between weeks 24 and 28 of the pregnancy. A woman is considered low risk if she does not have any of the factors and might not be checked at all unless her condition changes during the pregnancy.

Women may have one or more of the following glucose tests to diagnose gestational diabetes:

• Fasting plasma glucose test (FPG). A sample of blood is taken from a vein after the woman has not had anything to eat or drink (except water) for eight to 10 hours. The blood is tested for the amount of glucose (blood sugar) that is present in the sample. If the glucose level is 126 milligrams per deciliter (mg/dL) or higher on two occasions, the woman may be diagnosed with gestational diabetes.
  
  
• Random plasma glucose test (RPG). A sample of blood is taken from a vein and tested for the amount of glucose present in the blood. This is not done after fasting so the glucose in the blood may be high. However, the level should be less than 200 mg/dL. If the level is 200 mg/dL or higher on two occasions, the woman is considered to have gestational diabetes.
  
  
• Glucose challenge test. For this screening test, the woman drinks a sugary beverage (50 grams of glucose) and the blood is checked an hour later. This test may be done at any time during the day. If the glucose level is over 140 mg/dL, the results are considered positive (abnormal). Not all women with a positive screening test have diabetes.
  
  
• Oral glucose tolerance test (OGTT). This test is more sensitive than the FPG test and can often detect milder cases of diabetes. It measures blood glucose levels four or five times over a three–hour period. Prior to the test, the woman must not eat or drink anything but water for eight to 10 hours. Before starting the test, a blood sample is taken to provide a fasting blood glucose level. The woman drinks a sugary beverage (100 grams of glucose) and blood samples are taken every hour for three hours. Blood glucose levels are obtained from these samples and compared to normal levels.
  
  According to the American Diabetes Association (ADA), these levels represent the criteria for diagnosing gestational diabetes:
  
  

  Above-Normal Results for OGTT (100-gram glucose load)
  Fasting	95 or higher
  At one hour	180 or higher
  At two hours	155 or higher
  At three hours	140 or higher

If glucose levels are above normal at least twice during the test, gestational diabetes is diagnosed. The OGTT is usually performed between the second and third trimester of pregnancy when the greatest amount of insulin resistance occurs.

The fasting plasma test and random plasma test are often administered as the first test to check for gestational diabetes. However, these tests may not detect gestational diabetes in all women. Most physicians rely on additional tests to confirm the diagnosis.

It is important to remember that having a high glucose level on the glucose screening test does not necessarily mean the woman has gestational diabetes. For a clear diagnosis, the glucose tolerance test must show abnormal levels of blood glucose.

In addition to having these diagnostic tests, women who have gestational diabetes may undergo a fructosamine test. This blood test assesses glucose control over the past several weeks. It may be used in place of the glycohemoglobin test, which assesses glucose control over the past several months. Patients may also be advised to have fetal monitoring tests such as ultrasound.

According to some scientists, diagnosis of gestational diabetes can be more accurate using ultrasound to measure the midsection of a fetus, instead of relying on maternal glucose tests. These researchers propose administering insulin not to all hyperglycemic pregnant women but rather to those whose fetus has an abdominal size in the 75^th percentile or higher. However, this diagnostic and treatment protocol is not standard.