Glucose Tolerance Test (GTT)

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Extra Tests You May Be Offered

A Glucose tolerance test (GTT) is a way of examining how your body handles glucose. If a GTT is deemed necessary it is routinely performed at 26-28 weeks of pregnancy (earlier if you have had gestational diabetes previously).

If the result is not as expected you will be referred to Southmead Hospital Antenatal Clinic Diabetes Obstetric Team if you are booked to birth there. If you are booked at RUH or UHB, then a referral will be made to their endocrine service.

Why do I need a GTT?

During pregnancy some women develop gestational diabetes, which usually resolves after your baby is born, and a further blood test with your GP surgery will be taken six weeks after delivery to ensure this. It is important that this is recognised in order for the appropriate advice and treatment to be accessed, for the wellbeing of you and your baby.

Some people are more likely to develop gestational diabetes if they have:

  • A family history of diabetes
  • A body mass index of greater than 30
  • Delivered a larger than average weight baby
  • Had gestational diabetes in a previous pregnancy
  • Have a family origin with a higher prevalence to diabetes, such as being of South Asian, Afro-Caribbean or Middle Eastern decent.

What should I do before coming for the GTT?

As your midwife has identified a risk factor for gestational diabetes you will have been given an appointment for an oral Glucose Tolerance Test, usually at around 26/28 weeks of pregnancy. 

You will need to have two blood tests taken. The second blood test will be taken two hours after the first. You will need to remain within the clinic until the test is completed. If you walk around or go home the test will become invalid as you will be burning off the sugar you have been given.


  • To have nothing to eat or drink (except from plain water and regular medication but not any ant- acids) from 10pm the night before the test.
  • Following your first blood test you need to wait in the waiting area for two hours, until the second blood test. This is to ensure that you are resting, as any activity could alter the results of the test.
  • If you are a smoker then please refrain from smoking the midnight before the test until after the second blood sample has been taken, as smoking can interfere with the test.
  • Following the second sample you may continue with your day as normal.
  • You will be contacted by the Antenatal Clinic for the hospital you are booked with if the results are abnormal, usually within three working days.

For more information visit


Diabetes in pregnancy: Management of diabetes and its complications from pre-conception to the postnatal period, NICE (2008).