Written by:

Dr Diane Farrar

RM, BSc health science, BSc psychology, PhD reproductive endocrinology

Dr Diane Farrar is a practising midwife with over 25 years’ experience; her clinical experience has been mostly spent on the labour ward. Her primary position is as a senior Research Fellow at the Bradford Institute for Health Research, she is also a visiting Associate Professor at the University of Leeds and visiting Research Fellow at the University of York. Diane is lead for the Reproductive and Childbirth, Clinical Research Network team at the Bradford Women’s and Newborn unit. Diane’s research interests include gestational diabetes, blood pressure changes and hypertensive disorder in pregnancy and obesity.

The oral glucose tolerance test (OGTT) for gestational diabetes

In this article:

  • Why am I being offered an oral glucose tolerance test (OGTT)?
  • Why is the OGTT important?
  • What are the risks of declining the test if offered?
  • What is the oral glucose tolerance test (OGTT)?
  • What happens during the OGTT?
  • I've been diagnosed with GDM – What are the treatment options?
  • Main points
  • Further information:

The oral glucose tolerance test (OGTT) for gestational diabetes

Why am I being offered an oral glucose tolerance test (OGTT)?

Gestational diabetes Mellitus (GDM) can develop during pregnancy.

‘Screening’ is the process of identifying women who are at higher risk of GDM compared with other pregnant women. Once identified as at higher risk, those women can consider having a diagnostic test that will conclusively diagnose GDM (or not). Screening to find a higher risk population is done to avoid testing women who are unlikely to have GDM and reduce costs for the NHS.

In the UK, the National Institute for Health and Care Excellence (NICE) recommends only women at higher risk of GDM are offered diagnostic testing using a 75g oral glucose tolerance test (OGTT). Women at higher risk are those with one or more of the following characteristics:

  • Overweight with a BMI over 30 kg/m2
  • Previous large baby (4.5 kg)
  • Previous GDM
  • Family history of diabetes
  • Family minority ethnic origin with a high prevalence of diabetes (such as Asian, Black or Middle Eastern)

Sometimes maternity units may offer all women an OGTT. This may be because most women accessing their services are at an increased risk of GDM (have one of the above characteristics). In the United States, a glucose drink containing 50g of glucose may be offered to assess risk (a 50g glucose challenge test (OGCT)). After drinking this solution, a non-fasted blood sample is obtained and, depending on the glucose level, an OGTT may be offered.

Although the purpose of identifying risk characteristics is to understand the likelihood a woman will have of developing GDM, it is important to understand that some women without a risk characteristic will develop GDM, while some women with a risk characteristic will not develop GDM. Indeed research shows that risk characteristics and also the OGCT (the smaller 50g non-fasted screening test) are not great at identifying women who will develop GDM, and that is another reason why some maternity units may offer all women the OGTT. Because women are monitored throughout pregnancy, it is likely that women who are not offered an OGTT but who do develop GDM will be identified because of their symptoms. These symptoms of GDM include: increased thirst, increase water around the baby and a larger than normal baby. If you do develop these symptoms, your doctor or midwife may offer an OGTT, even if you have had a negative OGTT earlier in pregnancy.

Why is the OGTT important?

GDM is a condition similar to type 2 diabetes, but it only happens in pregnancy, and once the baby is born it usually disappears. GDM can happen because, as pregnancy progresses, a woman’s body becomes more resistant to insulin – this happens because of the hormonal changes of pregnancy. Risk characteristics (such as being overweight) can also increase insulin resistance, making GDM more likely. Some insulin resistance is normal and this happens to ensure the baby gets enough glucose and nutrients to grow and develop properly. If insulin resistance is too much and not enough insulin is produced to ‘manage’ that glucose (convert it to glycogen for storage in the liver and as fat), blood glucose levels will be higher than normal. Higher levels of glucose in the mother’s blood can lead to greater amounts of glucose passing to the baby, and this increases growth and the laying down of fat, which can lead to complications (see below). Pregnant women are, therefore, tested to see what their glucose levels are; if they are higher than normal, treatment will be offered that aims to reduce glucose to a normal level (see our page on treating gestational diabetes).

What are the risks of declining the test if offered?

If you decide not to have an OGTT, it will not be possible to assess your blood glucose levels. If you have high levels, these may go unrecognised, although you may develop symptoms. We know that GDM can lead to a greater risk of having a larger baby and this can lead to problems around the time of birth such as caesarean birth and shoulder dystocia (this is when baby’s shoulders have difficulty moving through the pelvis during birth). Research evidence shows that treatment to reduce glucose levels reduces the risk of having a large baby.

What is the oral glucose tolerance test (OGTT)?

The OGTT is a sugary drink. In the UK, a 75g test is used – this just means the drink contains 75g of glucose; other amounts are used in other countries. In the US, for instance, a 75g or a 100g OGTT can be used to diagnose GDM. Several small trials have been conducted to assess the use of chocolate bars and meal replacements in place of the OGTT, but these trials do not provide convincing evidence that replacements of the OGTT are any more effective. Furthermore, the use of meal replacements may be difficult to organise in a busy clinic setting.

This test ‘challenges’ a woman’s body to process the glucose in the drink, if the body can process the sugar and store it properly, the OGTT will be negative. However, if it cannot, a higher level of glucose than normal will be found (because the body will be unable to convert and store all the glucose), and the test will be positive.

What happens during the OGTT?

The OGTT will generally be offered to pregnant women with one or more of the risk factors described above. The presence of risk characteristics are usually assessed at the pregnancy booking appointment. If any one characteristic is present, the OGTT will be offered and arranged so that it happens sometime between 24 and 28 weeks of gestation. The test is conducted at 24 and 28 weeks of gestation because, by this time, pregnant women struggling to convert all their glucose and store it will be showing signs of insulin resistance and inadequate insulin production (higher than normal blood glucose levels). If a woman has had GDM in a previous pregnancy, an OGTT may be offered early in pregnancy.

An appointment date and time with instructions about what to do before the OGTT will be provided by the hospital you are receiving care from. The instruction/information sheet will explain how best to prepare. These are the basic steps you should take before attending your OGTT:

  • Eat as you do normally (your usual diet) in the days before your OGTT appointment
  • Take any prescribed medications as normal
  • Fast the night before you attend your OGTT appointment. This means not eating anything after midnight, although you will usually be allowed to drink water or black tea or coffee. You should not drink any sugary drinks or milk.

When you arrive at your OGTT appointment (which will usually be first thing in the morning):

  • A blood sample will be taken.
  • You will be asked to drink a sugary solution containing 75 g of glucose (you should remain seated as much as possible in the clinic once you have had this drink, because moving around uses energy and this may affect the results of your blood test). Some women may experience nausea and vomiting after drinking the sugary solution.
  • Further blood samples will be taken after 1 or 2 hours (or both).

The clinic staff will tell you when your test is complete. You can then leave and eat and drink normally. You may want to take a light snack with you to the clinic to eat after your test has completed and before you make your way home – after all, you have not had anything to eat since the previous evening and it will be lunchtime by the time you head off home.

The test results may take a few days to come through. Your doctor or midwife will be able to tell you the results at your next clinic visit. If your test results show that you have GDM, you may be telephoned and asked to attend the clinic before your scheduled appointment.

I’ve been diagnosed with GDM – What are the treatment options?

Treatment of GDM includes diet and physical activity adjustment (to control carbohydrates and reduce glucose availability), tablets called metformin (to improve insulin resistance) or insulin (to help glucose conversion and storage). For many women, diet and physical activity adjustment will be enough to control blood sugar.

Main points

  • Screening is the process of identifying women who are at higher risk of developing gestational diabetes in pregnancy.
  • Once a woman is considered to be at a higher risk, they are offered a diagnostic test – the oral glucose tolerance test (OGTT). To reduce costs for the NHS, the test is only offered to women who are at an increased risk.
  • Whether a woman is at high risk of gestational diabetes is determined by looking at several risk characteristics, including whether the woman has a BMI of over 30 or a family history of diabetes. Risk factors are usually assessed during the booking appointment.
  • However, it’s important to understand that studies have shown assessing risk characteristics is not an effective way of predicting gestational diabetes; women without any risk characteristics may still develop gestational diabetes, while women with risk characteristics may not develop it. This is why some maternity units offer all women the OGTT.
  • If you develop the symptoms of gestational diabetes, which include increased thirst and increased water around the baby, you should be offered an OGTT, even if you’ve previously had a negative OGTT earlier in pregnancy.
  • Because gestational diabetes can lead to complications, detecting the condition is very important.
  • The OGTT involves the woman consuming a sugary drink that contains 75g of glucose. This will test how well the body can process and store sugar.
  • The test is usually offered and arranged between 24 and 28 weeks of gestation. If you’ve had the condition in a previous pregnancy, it may be offered earlier.
  • You will be asked to fast the night before your test. When you arrive at your appointment, a blood sample will be taken before you’re given the sugary solution and then again 1 hour after you’ve drunk it. You’ll be informed of the result at your next clinic visit.
  • Treatment options for gestational diabetes include adjustments to diet and physical activity. These adjustments alone are usually enough to control gestational diabetes, but medications are also available.

Further information:

Farrar D, Duley L, Dowswell T, Lawlor D (2017). Different strategies for diagnosing gestational diabetes to improve maternal and infant health. Cochrane Database of Syst Rev Issue 8: Art. No.: CD007122. DOI: 007110.001002/14651858.CD14007122.pub14651854.
Tieu J, McPhee AJ, Crowther CA and M. P (2014). “Screening and subsequent management for gestational diabetes for improving maternal and infant health.” Cochrane Database Syst Rev Issue 2. Art. No.: CD007222. DOI:10.1002/14651858.CD007222.pub3.
Farrar D, Simmonds M, Bryant M, Lawlor D A, Dunne F, Golder S, Tuffnell D, Sheldon T A (2017). Risk factor screening to identify women requiring oral glucose tolerance testing to diagnose gestational diabetes: A systematic review and meta-analysis and analysis of two pregnancy cohorts. PLoS ONE 12(4): e0175288.
Farrar D, Fairley L, Wright J, Tuffnell D, Whitelaw D, Lawlor D A (2014). Evaluation of the Impact of Universal Testing for Gestational Diabetes Mellitus on Maternal and Neonatal Health Outcomes: A Retrospective Analysis. BMC Pregnancy and Childbirth doi:10.1186/1471-2393-14-317.
Farrar D (2016). Hyperglycemia in pregnancy: prevalence, impact, and management challenges. Int J Women’s Health 8: 519-527