Testing for gestational diabetes

If you are at risk of developing gestational diabetes, you’ll usually be offered an oral glucose tolerance test (OGTT).

There are some factors that increase your risk of gestational diabetes. Your midwife will ask you about these at your first antenatal (‘booking’) appointment, which happens around 8-12 weeks into your pregnancy. 

If you have any risk factors, you’ll be offered a test for gestational diabetes. This will usually happen when you're between 24 and 28 weeks pregnant, or even sooner if you’ve had gestational diabetes before. 

You may also be offered a test for gestational diabetes if:

"I wasn’t skinny, but I wasn’t massively obese either… I had no symptoms whatsoever. I had no expectation that the test would be anything other than a formality."


What is the oral glucose tolerance test (OGTT)?

The main test for gestational diabetes is the oral glucose tolerance test (OGTT). You might hear it called a glucose tolerance test, or a gestational diabetes test. The test is simple and will not harm you or your baby. You may have the test at your usual clinic, or at a special diabetes clinic.

If you’ve had gestational diabetes before you may be offered a kit so that you can check your own blood sugar levels from early pregnancy, instead of having an OGTT.

What does a glucose tolerance test involve?

The test involves two blood tests – one for before and the other for after having a sugary drink. This will show how well your body processes the sugars in the drink.

Step 1: You will be asked not to eat or drink anything, other than water, from midnight before the test. This is known as fasting.

Step 2: A nurse will take a blood sample to measure your fasting blood sugar level.

Step 3: You’ll drink a sugary drink.

Step 4: After 2 hours, you’ll have another blood test to measure your blood sugar level.

Your blood sugar level will rise after you have the sugary drink, and should then return to normal as your body processes the sugar.

You will be diagnosed with gestational diabetes if either:

  • your fasting blood sugar level was higher than expected, or
  • your blood sugar level hasn’t fallen enough after 2 hours.

It’s a good idea to bring a snack with you, as you won’t have eaten since the day before and will be hungry afterwards. You may also want to take someone with you, in case you feel dizzy.

What do the glucose tolerance test results mean?

The results of the OGTT show your plasma glucose levels in mmol/l. This shows how many millimoles of sugar there are per litre of blood. The higher the number, the higher your blood sugar.

You’ll usually be diagnosed with gestational diabetes if either of these apply:

  • your fasting plasma glucose level is 5.6mmol/l or more, or
  • your 2-hour plasma glucose level is 7.8mmol/l or more.

After the test

Gestational diabetes can develop at any time during pregnancy. So, even if the OGTT shows that you don’t have gestational diabetes, tell your midwife if you get any symptoms. Try to trust your instincts, and talk to them if you are worried.

If you’ve had gestational diabetes before and your OGTT result in early pregnancy is normal, you’ll be offered another OGTT at 24-28 weeks. This will show if you’ve developed gestational diabetes since the first test.

Do I have to get a test for gestational diabetes?

You don’t have to have the test, but there are a few things to keep in mind.

  • If you’ve been offered a test, this means your midwife thinks you could be at risk of gestational diabetes. If it is not managed, there is a small increased risk of serious complications
  • If you are diagnosed with gestational diabetes, you will be offered more care during your pregnancy and labour, to help reduce the risk of problems. People often say they find this reassuring.

Gestational diabetes doesn’t always need to be treated with medication or injections. It can often be managed by changing your diet and doing more exercise.

If changes in diet and exercise aren’t enough, gestational diabetes can usually be managed well with insulin or other medication.

The thought of having gestational diabetes can be a worry, but most people have healthy babies, as long as their condition is detected and then managed well.

Learn more about gestational diabetes.

Royal College of Obstetricians & Gynaecologists, Diabetes UK (2021). Gestational diabetes - Information for you. Available at: https://www.rcog.org.uk/media/b10mqyfw/pi-gestational-diabetes.pdf (Accessed January 2024) (Page last reviewed 09/2021)

National Institute for Health and Care Excellence (2020). Diabetes in pregnancy: management from preconception to the postnatal period. NICE guideline 3. Available at: https://www.nice.org.uk/guidance/ng3 (Accessed January 2024) (Page last reviewed 16/12/2020)

Review dates
Reviewed: 29 February 2024
Next review: 28 February 2027