Glucose Tolerance Test (GTT)

Glucose Tolerance Test (GTT)

What is a glucose tolerance test?

A glucose tolerance blood test (GTT) aims to detect diabetes caused by pregnancy, known as gestational diabetes. About 2.6% of all pregnant women have diabetes (10% of which already had diabetes before the pregnancy, and 90% developing it as a result of pregnancy).

Essentially, diabetes occurs when sugar (or glucose) accumulates in the bloodstream, known as hyperglycemia. The pancreas is unable to produce enough insulin to carry the glucose eaten from foods into the body's cells and muscles to be used for energy. If there is insufficient insulin, the sugar remains in the person's bloodstream, accumulating to high levels and causing the physical signs of diabetes.

Pregnancy places an added strain on the pancreas as your body as to produce extra insulin to meet your baby’s needs, especially from week 20 to 24 during the most rapid phase of growth. If your body is not able to keep up, it is likely you will develop gestational diabetes. However gestational diabetes is a temporary diabetic condition, and once the placenta is delivered, blood sugar levels return to normal within 24 hours.

Women who develop gestational diabetes, however, are generally more likely to have gestational diabetes with a subsequent pregnancy (but this is not always the case), and are at increased risk of developing diabetes later in their life.

Babies of diabetic women are not born with diabetes. However, due to a genetic inheritance, they may also be at a slightly increased risk of developing diabetes later in their life. In most cases, all you need to do to keep the diabetes under control is to follow a healthy diet and exercise program and test your blood regularly. In about 10 to 25 per cent of cases insulin injections are required control the condition.

The main health risk for pregnant women with uncontrolled gestational diabetes is producing an abnormally large baby (over 4.5 kg or about 10lb), called fetal macrosomia or large for gestational age (LGA).

What do the tests involve?

Some healthcare providers prefer to give all women a GTT, and others choose only to recommend the test for women who have a risk factor such as age, weight or previous history of diabetes.

If you are recommended to have a test, it is likely to be given when you are between 26 and 28 weeks pregnant, unless you’ve had gestational diabetes before, in which case your doctor will recommend that you are tested earlier.

Oral GTTs involve quickly drinking a sweetened liquid (glucola), which contains 50g of glucose. The body absorbs this glucose rapidly, causing blood glucose levels to rise within 30 to 60 minutes. A blood sample will be taken from a vein in your arm about 60 minutes after drinking the solution. The blood test measures how the glucose solution was metabolized (processed by the body).

Who is most at risk of gestational diabetes?

There are several factors that can put you at greater risk of developing gestational diabetes.

  • Being obese (with a BMI greater than 30).
  • Previously having a large baby weighing 4.5kg or more.
  • Previously having gestational diabetes.
  • A family history of diabetes (eg: a parent, sibling or child with diabetes).
  • A family origin with a high prevalence of diabetes, e.g being of South Asian, Polynesian, Middle Eastern or Asian descent.

What if I have gestational diabetes?

Most women who develop gestational diabetes during pregnancy give birth to normal, healthy babies and their blood sugar levels return to normal after the pregnancy.

It is recommended that you have another glucose tolerance test at around 6 to 8 weeks after your baby is born to check that your blood sugar levels have returned to normal.

However, it's estimated that up to 20 per cent of women with gestational diabetes actually already had undiagnosed type-2 diabetes before they were pregnant. Also women who have had gestational diabetes are at risk of developing type-2 diabetes after pregnancy.

To minimise the risk of this happening, you will be advised to keep to the special diet you were prescribed during pregnancy for the rest of your life, and you may be tested regularly to see if you are developing the condition You can also reduce your chance of developing type-2 diabetes by breastfeeding your baby.

Chandler, K. (2015, July 22). Glucose tolerance test. Retrieved from


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