Glucose Tolerance Test – and what the results mean

If you've been told you've got to have a GTT, here's what you need to do, what to expect on the day and how this could indicate gestational diabetes

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The Glucose Tolerance Test (aka the GTT – not to be mixed up with a G&T) is a test carried out during pregnancy to detect whether you may have Gestational Diabetes (GD).

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Does everyone have a Glucose Tolerance Test during pregnancy?

No, not everyone who’s pregnant has one. You’ll only be offered one if your midwife thinks there’s a likelihood you could develop Gestational Diabetes during your pregnancy.

“Not every hospital routinely carries out glucose tolerance tests,” says independent midwife Pam Wild, “but you can ask for one if you are worried and you think you need to be tested.”

While most tests are carried out between 26 and 28 weeks, sometimes you may be given the test around 16 weeks, depending on your medical history.

Main reasons your midwife or GP will recommend you take a Glucose Tolerance Test (GTT) are: 

  • you have previously had diabetes in pregnancy
  • you have a history of diabetes in your immediate family
  • if you have previously had a very big baby (4.5kg or heavier)
  • if they have found sugar or ketones in your urine
  • for pregnant women of certain ethnic origins (Black Caribbean, South Asian and Middle Eastern)
  • if your BMI (body mass index) is 30 or above

How does the Glucose Tolerance Test work?

A glucose tolerance test (GTT) or oral glucose tolerance test (OGTT) measures your blood glucose levels.

In the test, your blood is taken, and you’re then given a glucose drink. After 2 hours your blood is taken again to see how quickly your body is able to clear the glucose from your blood. That gives an indication of whether you’re showing signs of GD.

How do I take a glucose tolerance test?

What to do the night before your GTT…

  1. Make sure you have a good meal as you’ll have to fast from midnight and won’t be able to eat anything until after the test – although you can still drink water.
  2. The test itself takes two hours so prepare some reading materials.
  3. Pack a snack or some food to eat after your test as you’ll probably be very hungry.
    – “The worst thing for me was feeling incredibly faint afterwards as i obviously hadn’t had breakfast that morning so by the time i was able to eat again it was 11:30,” says MFMer BabyVogue. “Bear in mind i hadn’t eaten since 10pm the night before. I felt so faint and weak I could hardly talk. i walked to work straight after having the test as the hospital is only 10 mins walk away and by the time i got to my desk i was so weak I couldn’t speak to the guy who sits next to me. I then proceeded to eat a ridiculous amount of food but it took HOURS before I felt back to normal again. My body really didn’t like being without food for so long! I was 26 wks when I had the test so I guess bubs was having a bit of a growth spurt.”
  4. If you have other children, you may need to make childcare arrangements or have the daybag ready if your little one is keeping you company.
  5. If you are nervous or squeamish about needles, you might want to take your partner or friend with you.

Things to avoid before having a GTT

There are a few things you shouldn’t have or do before your GTT test:

  • Indigestion remedies
  • Chewing gum
  • Vitamins – but you can take any essential medicines as usual (check with your GP or nurse)
  • Smoking can affect the result of your GTT so you shouldn’t smoke from your fasting period and until the test is over

What happens on the day of the GTT…

  • The test will be done either at your local hospital or GP surgery.
  • First you’ll have a blood sample taken from your arm to measure the amount of sugar in your blood after fasting.
  • Next you’ll be asked to drink a very sweet mixture (often one called Polycal) or an energy drink like Lucozade, and some water (basically the equivalent of 75g glucose) slowly over 5 minutes.
    – “The stuff I had to drink was fine. It was kind of like the too-strong, squash-type drinks you get from coffee vending machines. I was surprised at how much of it I had to drink though so maybe mine was watered down quite a bit. If you are worried about the taste you could always ask to dilute it more,” says Baby_B.
  • You’ll then have to wait for two hours or more, either in the hospital or go home. You can drink water but can’t eat.
    – “The main thing is that the 2 hour wait is dull because you aren’t allowed to walk around,” says Baby_B.
  • After 2 hours, the nurse will take your blood again to see what the reading is compared to your baseline test.

“I couldn’t eat anything after 8pm the previous evening and only drink water from then on,” says mama_milf. “I had my first bloods taken from one arm at about 10.30 am and then had a measured amount of glucose (lucozade!) and then waited for 2 hours then have my last bloods taken from the other arm.”

“I had to then drink a small glass of what tasted like watered down icing sugar within a 10-minute window,” says berly153. “That down, I just relaxed for the next 3 hours reading mags and had a snooze.”

What if I have a phobia of needles?

Not good news. As you’ve probably already discovered, having your blood taken is part of pregnancy. “I’m afraid there are no alternatives,” says midwife Anne Richley, “but ultimately it’s your choice. Do speak to your midwife about each test, because, like with any fear, if you understand them and why they matter for you and your baby, you may feel braver.”

How long does it take to get the results of my GTT?

You should normally get your results back within a week – if you test positive for gestational diabetes. However, the timing does vary from area to area. If you have a normal reading, your midwife will tell you at your next antenatal appointment.

What do the results of a Glucose Tolerance Test mean?

The test measures the glucose levels in your blood. Blood glucose is measured in millimoles of glucose per 1 litre of blood (mmol/l). According to Diabetes UK:

It’s unlikely that you have gestational diabetes if:

  • Your first blood glucose count (after fasting): is below 6 mmol/L
  • Then after 2 hours: is below 7.8 mmol/L

You may have gestational diabetes if: 

  • Your first blood glucose count (after fasting): is over 7.0 mmol/L
  • Then after 2 hours: is over 11.0 mmol/L

Depending on how high your blood glucose levels are, your midwife or doctor will recommend further testing or a treatment plan for you. “I had the test when I was 28 weeks and it was positive,” says laurelladie. “It was 7.8 when the cut off was 7.7.”

A high result doesn’t necessarily mean you do have GD, but it will mean you need more tests.

“If the results show the levels are elevated, then you will need further screening,” explains midwife Pam. “This would be a HbA1C1 test which can tell us how the body is processing glucose over past month, so it is very accurate and not just a one off-reading like the initial test.”

What will happen if tests show I have gestational diabetes?

If GD is diagnosed, you’ll be given diet sheets and advice to help you keep your gestational diabetes until control. You’ll receive info on which foods will keep your blood sugar levels stable. You will also be given equipment to test your blood at home so you can keep an eye on your glucose levels.

If you have particularly unstable blood sugar, or if an ultrasound scan has shown that your baby is very large, you may be prescribed medication, or insulin injections.

One MFMer sep2612, who was diagnosed for gestational diabetes, says, “They were very quick to refer me to the diabetic clinic and had me straight back in the next day for a growth scan which showed that the baby had a slightly larger than average abdomen circumference (which is one of the big worries). They got me straight on insulin and metformin which I thought was a bit extreme to start with but a scan and appointment 4 weeks later revealed that the meds plus my careful diet had controlled things and everything was back to normal.”

What exactly is gestational diabetes?

Gestational diabetes is a condition that happens in pregnancy (so it’s related to, but not the same as, diabetes), It happens when your pregnancy prevents your body from making enough insulin, causing your blood sugar levels to be higher than usual.

Gestational diabetes only develops in pregnancy and goes away as soon as your baby is born, although it may raise your risk of developing type 2 diabetes later in life.

How common is it?

According to the NHS, it affects 1 in 25 pregnant women in the UK – which is 4% of all pregnancies.

Will I have ongoing appointments and care for my gestational diabetes? 

You will probably have longer and more frequent antenatal appointments with your midwife to keep tabs on your blood sugar readings, and you and your baby’s general well-being, particularly your baby’s growth.

Could gestational diabetes harm my baby?

If the condition is diagnosed and monitored properly, then most women with gestational diabetes go on to have normal pregnancies.

However, if it is not detected or managed properly, gestational diabetes does pose serious risks to both mum and baby. Potential complications include:

  • your baby growing too large resulting in a difficult birth
  • breathing or heart problems for your baby after delivery
  • premature delivery
  • miscarriage or stillbirth

But gestational diabetes can be well managed once it’s diagnosed. “The good news is that, with expert care from medical staff, your pregnancy and birth should both go smoothly,” says midwife Anne Richley,

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