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Gestational diabetes tests

If your healthcare team feel it’s important to test you for gestational diabetes, you’ll normally be tested between 24 and 28 weeks of pregnancy, although you might test earlier if you’re at higher risk of developing it or have had it before. Read on for more information. 

What tests determine gestational diabetes? 

The main test used to diagnose gestational diabetes is a blood test known as the oral glucose tolerance test (OGTT). This is a simple test that doesn’t harm you or your baby. 

Oral glucose tolerance test (OGTT) 

The OGTT takes just over two hours to complete and is usually done at the clinic or hospital where you have been receiving antenatal care, or a specialist diabetes clinic. 

How does an oral glucose test work? 

The test consists of the following steps: 

  1. You'll need to fast (not eat or drink anything except water) for 8-10 hours the night before and the morning of the test. 
  2. You’ll have a blood test to measure your blood glucose level. 
  3. You’ll then be given a glucose (sugary) drink. 
  4. After resting for two hours, you'll have another blood test to see how your body is dealing with the glucose. 

You shouldn’t eat anything before or during the gestational diabetes test. But it’s a good idea to bring a snack with you to have afterwards, as you’ll probably be hungry. You can have sips of water to drink before and during the test.  

My midwife referred me to the hospital to do an Oral Glucose Tolerance Test (OGTT). I didn’t think I needed to worry about diabetes as I wasn’t overweight, but my South Asian background was a risk factor for gestational diabetes, which I hadn’t realised up to this point. 

Read Reena’s story of her experience of having gestational diabetes. 

After the test 

You will be diagnosed with gestational diabetes if your fasting blood sugar level is 5.6mmol/l or above or if your 2hr post glucose blood sugar level is 7.8mmol/l or above.  

If you are diagnosed with gestational diabetes you should be referred to a joint specialist diabetes and antenatal clinic for your care.  

Gestational diabetes can develop at any time during pregnancy. So if you develop any symptoms (despite the OGTT showing that you don’t have gestational diabetes), it’s important to talk to your midwife about this, or anything else you’re concerned about. 

So even if the OGTT shows that you don’t have gestational diabetes, talk to your midwife if you go on to develop any symptoms. It’s important to trust your instincts and tell your health professional if there is anything that you are worried about. 

How do I prepare for a gestational diabetes test? 

Apart from fasting for 8-10 hours before the test, you don’t need to do anything special to prepare for the gestational diabetes test. But check the leaflet you are given about the test so you’re sure what to expect from your clinic.  

Is 30 weeks too late for a glucose test? 

If you have symptoms of diabetes, or any concerns at any point during your pregnancy it is important to discuss this with your healthcare team. This can help to ensure that a care plan is in place for the rest of your pregnancy and birth of baby.  

The most important thing is identifying that you have or don’t have gestational diabetes, regardless of when the test is done. This usually happens between 24-28 weeks of pregnancy but can happen later.  

What happens if you are diagnosed with gestational diabetes? 

Once you’ve been diagnosed with gestational diabetes, your specialist antenatal and diabetes healthcare team will recommend the best treatment for you and advise you on how you can test your blood sugars at home and follow a healthy diet.  

Some women might be able to reach their target blood sugar levels with diet and exercise alone, or you might be prescribed metformin or insulin to help. 

Read more about what care you can expect before and after you give birth if you have gestational diabetes, and what treatments are available.  

Next Review Date
Content last reviewed
30 March 2023
Next review due
30 March 2026
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