Testing for diabetes during your pregnancy – The Glucose Tolerance Test (GTT)

Written by

Rob Buist

Published on


STOP PRESS 6th April 2020


Because of the latest Coronavirus crisis I have decided that you should not be sitting in a laboratory waiting room for two hours as I think this will present an unacceptable risk in these troubling times.
We have just received guidance from Queensland Health (I know) and the following approach is also being taken in NSW:

All women:

Instead of performing a full GTT (see original advice, below) we will simply perform a fasting blood sugar / glucose (FBG) level at 26 to 28 weeks gestation.

  • If your FBG is 4.6 or less you do not have Gestational Diabetes (GDM)
  • If your FBG is between 4.7 and 5.0 you will need to undergo a GTT as per the information below
  • If your FBG is 5.1 or more you have GDM and we will manage accordingly

By fasting we mean that you should have nothing to eat from midnight prior to the test. You can have some water and if you are taking regular medicines in the morning please do so. We will schedule your blood test for as early as possible in the morning.

Women who have had Gestational Diabetes (GDM) in a previous pregnancy, or have other risk factors for GDM:

We will perform a fasting blood glucose (FBG) level and a blood test commonly used in diabetes management called an HBA1C at 11 to 13 weeks gestation. If either of these tests is abnormal we will have made a diagnosis of GDM and we will progress accordingly.

We will perform all of these tests at my office in order to keep you away from pathology centres as much as possible.

Stay safe.


Somewhere around 5-12% of pregnant women will develop a mild form of diabetes during their pregnancy. This is called gestation diabetes mellitus (GDM) and it usually resolves after the birth of the baby. GDM arises because a number of hormones produced by the placenta increase the mother’s blood sugar levels; this is a normal process that is designed to ensure that the baby is appropriately nourished. However in some women these placental hormones cause the mother’s blood sugar to rise excessively causing the mother to be affected by GDM. Women with GDM often have no symptoms or risk factors for diabetes. To diagnose – or rule out – GDM you have a special blood test (actually tests) that needs to be performed at the beginning of the third trimester, i.e. around 26-28 weeks. This timing relates to the growth of the placenta and the increasing amounts of hormones it produces.

If you have undetected increased blood sugar levels (i.e. GDM) during your pregnancy a number of complications can occur for yourself and your baby. These mainly relate to the fact that – as you might have guessed – lots of sugar can make the baby grow bigger than it needs to. This increased baby size can increase your chances of an induced or caesarean birth. The baby can be affected – paradoxically, I know – by low blood sugar levels after the birth which, in turn can lead to the baby being admitted to the Special Care Baby Unit (SCBU) after the birth.

Correctly diagnosing and treating GDM can prevent these complications. Diagnosing GDM is also beneficial to mothers as having GDM is a sign that you may be at risk of developing Type 2 Diabetes later in your life and you may wish to adopt lifestyle changes that help you to prevent that condition from occurring – you know, all the boring stuff, exercise and eating healthily.

Unfortunately GDM causes no symptoms and it cannot be diagnosed by a simple blood or urine test. Instead you need to have a Glucose Tolerance Test (GTT). The GTT is designed to see how your body responds to a load of sugar.

The GTT:

  • You will need to phone a pathology laboratory to book a GTT. Some pathology centres require you to follow a special diet for 3 days. This is not necessary in pregnancy; just follow your usual diet.
  • Allow 2-3 hours for the test to be completed and plan a quiet morning afterwards, as some women will feel a little nauseated and you need time for breakfast once the test is completed.
  • The test is performed early morning. You are required to fast from midnight, but water and usual medications are permitted.
  • On arrival at the pathology centre a blood sample will be collected.
  • Following this, you will need to drink a sweetened drink (it’s like lemonade with extra sugar) and then sit quietly for two hours.
  • Blood samples will be collected one and two hours after consuming the drink to assess your body’s response to the glucose load.
  • The test is then completed.
  • You may wish to bring a book or iPad to pass the time.

As I said above this test should be performed at around 26 to 28 weeks. However some women have higher risk factors for GDM than others and will need an earlier test in pregnancy in addition to the third trimester test (lucky them!).

The higher risk factors for GDM are:

  • Having previously elevated blood glucose levels including GDM in a previous pregnancy
  • Being less than 15 years younger than your Obstetrician (i.e. over 40 years old)
  • A family history of diabetes, including GDM
  • Being more overweight than your Obstetrician (i.e. having a Body Mass Index (BMI) of more than 30)
  • Having given birth to a previous large baby
  • Having Polycystic Ovarian syndrome (PCOS) especially if Insulin Resistance is present
  • Being on certain medications.

You will be contacted with your results. Should your result be abnormal DO NOT PANIC- it is not the end of the world. You will be referred to a specialist with an interest in GDM. In addition you will be referred to a specialist diabetes midwife who will:

  1. Explain your diagnosis and give you advice about lifestyle factors that can help reduce your sugar levels (mainly diet and exercise).
  2. Teach you how to monitor you blood sugar levels.

After your baby is born another GTT will be arranged in order to make sure that your GDM has completely resolved. In the vast majority of women it does.

This information sheet was written by my Midwifery colleague Amanda Bartlett. Amanda is a Diabetes education specialist. As always I take full responsibility for any errors or omissions.

Rob Buist November 2016