Glucose tolerance test (GTT)

What is a glucose tolerance test?

A glucose tolerance test (GTT) diagnoses diabetes in pregnancy by checking how well your body regulates your blood sugar levels.

Gestational diabetes (GD) is a common pregnancy complication. It’s thought to affect about one pregnant woman in 10 (AIHW 2017).

Testing for GD is offered to all pregnant women in Australia (ADIPS 2014, DH 2018, RANZCOG 2017). Usually, you’ll have the test when you're between 24 weeks and 28 weeks pregnant (ADIPS 2014, DH 2018, RANZCOG 2017).

Depending on your medical history, you may be offered the test earlier than this (ADIPS 2014, DH 2018, RANZCOG 2017). For example, your doctor or midwife will offer the GTT sooner if you've had gestational diabetes before.

Why do I need a GTT?

Gestational diabetes often doesn't cause obvious symptoms, which is why testing is important (Diabetes UK 2017, NHS 2016).

If GD isn't recognised and treated, it may put your health and your baby’s health at risk (NCCWCH 2015, RCOG 2013).

GD happens when your body doesn’t make enough insulin. Insulin is a hormone that keeps your blood sugar level stable. It also helps your body store sugar for when you need it later.

During pregnancy, your body has to produce extra insulin to meet your baby's needs, especially when he’s growing rapidly. If your body can’t make enough insulin, you may end up with too much sugar in your blood, resulting in GD.

Having too much sugar in your blood may mean that your baby grows large. This increases your chances of having an induced labour, and a caesarean birth (NCCWCH 2015, RCOG 2013).

Sadly, GD, especially if it’s not controlled, can raise the risk of a baby being stillborn (RCOG 2013), although this is rare. That’s why it’s so important to follow the advice of your doctor or midwife if you’re diagnosed with GD.

Am I at risk of developing gestational diabetes?

You're more likely to develop GD if:

• Your body mass index (BMI) is 30 or above.
• You have previously given birth to a large baby weighing 4.5kg or more.
• You’ve had GD before.
• You have a parent or sibling with diabetes.
• Your family origins are Aboriginal or Torres Strait Islander, Polynesian, Indian subcontinental, South or East Asian, Middle Eastern, Hispanic, African or Native American.

If you fall into one or more of these groups, your doctor or midwife may recommend you have a GTT as soon as possible after your first antenatal appointment, rather than waiting until 24 weeks (ADIPS 2014, DH 2018, RANZCOG 2017).

What happens during a glucose tolerance test?

A nurse or midwife will take three blood samples from you, each an hour apart. You may need to go to a hospital or clinic for it, or it may be done at your doctor's surgery.

The GTT needs to be done on an empty stomach to make sure the result is accurate. Usually you’ll need to fast overnight (NHS 2016, RCOG 2013). But your caregiver or nurse will confirm how long you need to avoid eating and drinking before you have the test.

You should be able to drink tap water during this time, but ask your caregiver, hospital or clinic to make sure (NHS 2016, RCOG 2013). If you're taking medication, check with your doctor that it's safe to take your usual dose while you're fasting.

When you arrive at the clinic, the hospital or your doctor’s surgery, a nurse or midwife will take the initial blood sample from a vein in your arm. The sample is used to measure your blood sugar level after you've been fasting. This is called a baseline test.

The nurse or midwife will then give you a sweet, sugary mixture, containing the equivalent of 75g of glucose (DH 2018, NCCWCH 2015). You need to drink all of the mixture.

The nurse or midwife will ask you to sit and rest for two hours, while you digest the sugary drink. You shouldn't eat or drink anything while you’re waiting. Even chewing gum is out, because it can affect your results.

After one hour, the nurse or midwife will take another blood sample. He’ll take another one after two hours. Your baseline and follow-up blood samples will be compared with the normal range for a pregnant woman (NHS 2016, SIGN 2010). Your caregiver should receive the results of the test within 48 hours.

The two-hour wait may be a bit of a drag, especially if you’ve got a child in tow. Bring a book, a magazine or something else to keep you (and your child) occupied during the two-hour test.

Some hospitals, clinics or surgeries may allow you to leave the premises, as long as you take it easy. Most prefer you to stay, though, as it can influence your follow-up sample if you’ve moved around too much.

It’s a good idea to take a snack with you to eat once the GTT is complete, especially if you're driving yourself home, as you're bound to be hungry.

What other tests are available for GD?

There are several ways to test for GD. Most of them involve taking glucose in a drink or snack and having a blood test (Farrar et al 2017). Australian guidelines recommend the GTT as the routine way to do it (DH 2018).

Your doctor or midwife may test your urine at each antenatal appointment. If there is sugar in your urine, this could be a sign of GD. But it's only a problem if the sugar is too high in one or more tests (NCCWCH 2015).

Your caregiver should offer you a GTT to make sure, as testing your urine isn’t the most reliable way to screen for diabetes (NCCWCH 2015).

What happens if I get a positive result from the GTT?

It may come as a shock to find out that you have GD, and there can be a lot to take in.

A health professional will discuss your results with you, and you may be referred to a dietitian or specialist (RWHM 2017).

Your treatment will depend on the level of sugar found in your blood (NCCWCH 2015).

Your caregiver or a dietitian will advise you about choosing food and drink that keep your blood sugar level stable. You’ll also be given a kit to test your blood glucose levels.

You may be able to keep GD under control simply by following a healthy diet and doing exercise (Diabetes UK 2017, NCCWCH 2015). Read more about a healthy diet for managing GD.

GD increases your risk of developing type 2 diabetes later in life (ADIPS 2014, NCCWCH 2015, NHS 2016). Any lifestyle changes you make now will help you lower this risk.

If you have gestational diabetes, you’ll have extra antenatal care, to check your health and your baby's progress (NCCWCH 2015, NHS 2016, RCOG 2013). Your appointments are likely to take longer.

You may also be offered extra ultrasound scans to check your baby's size (NCCWCH 2015, RCOG 2013, SIGN 2010).

If your blood sugar levels are very high or unstable, or if your ultrasound scan shows your baby to be large, you may need to give yourself insulin injections to control your blood sugar level (RWHM 2017). Your doctor or midwife will show you how to do an injection. Injecting insulin only takes a second, and though it may be a bit uncomfortable at first, it shouldn’t hurt.

Alternatively, you may need to take tablets (RWHM 2017).

Most women who develop diabetes during pregnancy give birth to healthy babies. Your midwife or doctor will check your baby’s blood sugar level soon after birth, as some babies have low blood sugar levels (NCCWCH 2015). Even if this affects your baby, his level should quickly return to normal (RCOG 2013).

You should be offered another glucose tolerance test, usually between six weeks and 12 weeks after the birth, to check that your blood sugar level has returned to normal (ADIPS 2014, RWHM 2017).

Find out more about gestational diabetes.