Gestational Diabetes Screening & Diagnosis

Gestational diabetes (GD), also known as gestational diabetes mellitus (GDM), is usually diagnosed using a glucose tolerance test (GTT), also known as an oral glucose tolerance test (OGTT), most commonly carried out between 24 and 28 weeks of pregnancy.

However, who is offered screening, when testing takes place and the blood sugar levels used for diagnosis can differ depending on where you live.

This guide explains who should be offered screening for gestational diabetes, what happens during a GTT, the levels used to diagnose GD across the UK and Ireland, and what other testing options may be available.

What is gestational diabetes?

During pregnancy, hormones produced by the placenta make the body more resistant to insulin. This means the body needs more insulin to keep blood sugar levels within a healthy range.(1)

Most people can produce enough additional insulin to meet this increased demand. Gestational diabetes develops when the body cannot produce enough insulin to overcome the increased insulin resistance, causing blood sugar levels to rise.(1)

Insulin resistance increases as pregnancy progresses, which is one reason gestational diabetes is most commonly diagnosed during the second or third trimester.(1)(2)

Does gestational diabetes cause symptoms?

Gestational diabetes often causes no noticeable symptoms, which is why screening is important.(3)

Some symptoms associated with high blood sugar, such as increased thirst or needing to urinate more often, can also occur during a normal pregnancy. Symptoms alone therefore cannot be used to diagnose or rule out gestational diabetes.

How common is gestational diabetes?

Gestational diabetes is one of the most common medical conditions affecting pregnancy.(4) Exactly how common it is can be difficult to measure because screening methods, diagnostic levels and the populations being tested differ.

Recent NHS data suggest that gestational diabetes is now considerably more common in England than older estimates suggested. A large study of more than 2.7 million births across 184 hospitals found that the proportion of pregnancies with a recorded diagnosis of gestational diabetes increased from 8.1% in 2018 to nearly 12% in 2022.(5)

Rates of gestational diabetes vary between populations and are influenced by several factors, including ethnicity, maternal age and BMI.(5)(6)

This variation is one reason why you may see very different figures quoted for how many pregnancies are affected by gestational diabetes.

Who should be offered screening for gestational diabetes?

In the UK, not everyone is routinely offered a GTT for gestational diabetes. In many areas, screening is based on whether you have one or more recognised risk factors.

At your booking appointment, your maternity team should assess whether you have any risk factors that mean you should be offered testing for gestational diabetes.

NICE recommends offering a GTT at 24–28 weeks if you have any of the following risk factors:

• a BMI of 30 kg/m² or above
• previously given birth to a baby weighing 4.5 kg (9 lb 15 oz) or more
• gestational diabetes in a previous pregnancy
• a parent, brother or sister with diabetes
• a family origin with a high prevalence of diabetes

These are the risk factors NICE uses to determine who should routinely be offered testing, but they are not the only factors associated with developing gestational diabetes. Some women who develop GD have none of these recognised risk factors.

Can you develop gestational diabetes without any risk factors?

Yes. Although recognised risk factors are used to decide who should routinely be offered screening in many areas, gestational diabetes can also develop in women who have none of these risk factors.

In a Gestational Diabetes UK survey of 3,270 women who had been diagnosed with gestational diabetes:

• 45.4% did not have a BMI of 30 kg/m² or above
• 11.5% reported having none of the recognised risk factors listed in the survey

This means risk-factor-based screening can miss some women who develop gestational diabetes.

Some countries and maternity services use universal screening, where all pregnant women are offered testing regardless of their individual risk factors. In the UK, however, approaches to screening can vary depending on where you receive your maternity care.

Other factors associated with gestational diabetes

There are also factors associated with an increased likelihood of developing gestational diabetes that are not included in the standard NICE risk-factor criteria for offering a GTT.

These include increasing maternal age and conditions associated with insulin resistance, such as polycystic ovary syndrome (PCOS).

Having one of these factors does not mean that you will develop gestational diabetes, just as having none of the recognised risk factors does not mean that you cannot develop it.

When is gestational diabetes testing carried out?

For most women who are offered testing because of recognised risk factors, a GTT is carried out between 24 and 28 weeks of pregnancy.

This timing is important because insulin resistance naturally increases as pregnancy progresses. Gestational diabetes may therefore develop later in pregnancy even if blood sugar levels were normal earlier on.

If you’ve had gestational diabetes before

If you’ve had gestational diabetes in a previous pregnancy, you should tell your maternity team as early as possible, as earlier blood sugar testing or monitoring may be recommended.

In England, Wales and Northern Ireland, NICE recommends offering either:

• self-monitoring of blood sugar levels from early pregnancy, or
• a GTT as soon as possible after your booking appointment.

If an early GTT is normal, NICE recommends another GTT at 24–28 weeks, as gestational diabetes can develop later in pregnancy.

Guidance and local pathways differ across the UK and Ireland, but previous gestational diabetes is recognised as an important reason for earlier assessment in a subsequent pregnancy.

Can gestational diabetes develop very early in pregnancy?

Gestational diabetes is most commonly diagnosed during the second or third trimester, but raised blood sugar levels can be identified much earlier in pregnancy.

When raised levels are found early, there can sometimes be an assumption that diabetes must have been present before pregnancy. Very high blood sugar levels that meet the diagnostic criteria for diabetes outside pregnancy require further assessment. However, milder hyperglycaemia in early pregnancy does not automatically mean that someone has pre-existing type 2 diabetes. Research recognises early gestational diabetes, sometimes referred to as early GDM, as a complex area where diagnosis and classification remain debated.(7)(8)

Can gestational diabetes be diagnosed after 28 weeks?

Yes. Gestational diabetes can be identified later in pregnancy, particularly if there are signs that suggest further investigation may be needed.

For example, your maternity team may consider testing or monitoring your blood sugar levels if glucose is repeatedly found in your urine or if concerns arise during your pregnancy that could be associated with gestational diabetes.

What if glucose is found in my urine?

Glucose (sugar) can sometimes be found in urine during pregnancy and does not automatically mean that you have gestational diabetes.

However, higher or repeated levels of glucose in your urine may be a reason for further testing. NICE recommends considering further testing to exclude gestational diabetes if routine antenatal urine testing shows glucose of 2+ on one occasion, or 1+ on two or more occasions.

A urine test itself cannot diagnose gestational diabetes.

What is a glucose tolerance test (GTT)?

A glucose tolerance test (GTT), also known as an oral glucose tolerance test (OGTT), measures how well your body processes glucose (sugar).

For gestational diabetes, the standard test is a 75 g GTT. This means you drink a glucose solution containing 75 grams of glucose after fasting overnight, and blood samples are taken to see how your body responds.

What happens during a GTT?

You will usually be asked not to eat or drink anything other than water for around 8–10 hours before your appointment. Your maternity team should give you instructions about when to stop eating and drinking.

When you arrive:

1. A blood sample is taken to measure your fasting blood sugar level.
2. You drink a glucose solution containing 75 g of glucose.
3. You remain at the clinic or hospital while your body processes the glucose.
4. Blood samples are then taken at set times after the glucose drink. Under the NICE pathway, a further sample is taken at 2 hours, although some maternity services also take a 1-hour sample.

You will normally be asked to remain resting during the test and not to eat, smoke or vape. Small amounts of plain water are generally permitted, but follow the instructions given by your maternity team.

The whole appointment usually takes a little over 2 hours.

What does the GTT show?

After drinking the glucose solution, your blood sugar level rises. Your body should respond by releasing insulin, which helps move glucose out of the bloodstream.

The blood samples show how effectively your body is managing this glucose load. If your fasting or post-glucose blood sugar level reaches the diagnostic threshold for gestational diabetes, you will be diagnosed with GD.

You do not need to have an abnormal result at both points for gestational diabetes to be diagnosed.

What GTT results diagnose gestational diabetes?

The blood sugar levels used to diagnose gestational diabetes are not the same everywhere. Different diagnostic criteria are used across the UK and Ireland, and some NHS Trusts use local testing pathways or take additional blood samples during the GTT.

This means the number of blood samples taken, and in some cases the levels used for diagnosis, may differ depending on where you receive your maternity care.

Only one result needs to meet or exceed the diagnostic threshold for gestational diabetes to be diagnosed.

England, Wales and Northern Ireland – NICE NG3

NICE recommends a 75 g, 2-hour GTT. Gestational diabetes is diagnosed if either of the following results is reached:

Blood test	Diagnostic level
Fasting	5.6 mmol/L or above
2 hours after the glucose drink	7.8 mmol/L or above

NICE NG3 is also endorsed for use in Northern Ireland. Some NHS Trusts and maternity services use local protocols that differ from, or include additional testing alongside, the standard NICE pathway. For example, some maternity services also take a 1-hour blood sample during the GTT.

Scotland – SIGN 171

Scotland uses the SIGN 171 guideline, which has different diagnostic thresholds:

Blood test	Diagnostic level
Fasting	5.3 mmol/L or above
1 hour after the glucose drink	10.6 mmol/L or above
2 hours after the glucose drink	9.0 mmol/L or above

The 1-hour measurement may not be taken by every maternity service.

Republic of Ireland – HSE guidance

In the Republic of Ireland, the following HSE thresholds are used for a 75 g GTT:

Blood test	Diagnostic level
Fasting	5.1 mmol/L or above
1 hour after the glucose drink	10.0 mmol/L or above
2 hours after the glucose drink	8.5 mmol/L or above

One or more results meeting or exceeding these levels is diagnostic of gestational diabetes.

Why are the diagnostic levels different?

You may notice that the thresholds vary considerably. For example, a fasting result of 5.4 mmol/L would meet the diagnostic threshold in Scotland and the Republic of Ireland, but would be below the NICE threshold used in England, Wales and Northern Ireland. Likewise, a 2-hour result of 8.0 mmol/L would meet the NICE threshold but would be below the current SIGN 171 threshold used in Scotland.

This can understandably be confusing. The differences exist because guidelines have adopted different diagnostic criteria and interpreted the available evidence in different ways. Local maternity services may also use their own testing pathways.

I explain why these thresholds differ, and the evidence behind them, below.

Where do the diagnostic levels come from?

Research has shown that there isn’t one exact blood sugar level where the risks associated with gestational diabetes suddenly begin. Instead, as blood sugar levels rise, the chance of certain pregnancy complications gradually increases.

One of the largest studies looking at this was the HAPO study, which included more than 23,000 pregnancies. It found that as blood sugar levels increased, so did the likelihood of outcomes such as having a larger baby, caesarean birth and low blood sugar levels in the baby after birth.(9)

The difficulty is that the research did not reveal one obvious level that could be used to say, “below this is normal, above this is gestational diabetes”.

A cut-off therefore has to be chosen for diagnosis, and different organisations have chosen different levels based on how they have assessed the evidence.

What if I cannot tolerate the GTT?

Not everyone is able to complete a GTT. Some people feel sick or vomit after drinking the glucose solution, while for others the test may not be suitable.

If you are unable to complete a GTT, speak to your maternity team about alternative ways of checking for gestational diabetes. Depending on your circumstances and local maternity pathway, you may be offered home blood sugar monitoring for a period of time instead.

GTT after bariatric surgery

A GTT may not be suitable if you have previously had certain types of bariatric surgery, such as gastric bypass or sleeve gastrectomy. The rapid absorption of a large glucose load can cause dumping syndrome, reactive hypoglycaemia and large swings in blood sugar levels.(10)(11)

Alternative screening using home blood sugar monitoring or continuous glucose monitoring (CGM) may be recommended instead. If you have had bariatric surgery, tell your maternity team before having a GTT so that an appropriate method of screening can be discussed.(10)

GTT with hyperemesis gravidarum (HG)

A GTT may be particularly difficult if you have hyperemesis gravidarum (HG), especially if you are struggling to keep food or fluids down. Research has also suggested that the effects of HG, including reduced food intake and weight loss, may affect glucose testing results.(12)(13)

If you have HG and are concerned that you will be unable to tolerate the glucose drink, discuss this with your maternity team before the test. They can advise whether the GTT should be attempted, delayed, or whether another approach to screening may be appropriate.

What if I vomit during the GTT?

Feeling nauseous during a GTT is quite common, particularly when drinking the glucose solution on an empty stomach.(14)

If you vomit during the test, tell the staff looking after you. The GTT usually cannot be interpreted as planned if the glucose drink has not been retained. Your maternity team can then discuss whether to repeat the GTT or use an alternative method of testing.

Can HbA1c be used to diagnose gestational diabetes?

HbA1c gives an indication of average blood sugar levels over the previous 2 to 3 months. Although widely used to diagnose and monitor diabetes outside pregnancy, it has important limitations during pregnancy and should not be used to rule out gestational diabetes.

Pregnancy affects red blood cell turnover and blood volume, which can affect HbA1c results. HbA1c also gives an average and may not show the rises in blood sugar after meals that are important in gestational diabetes. A normal HbA1c therefore does not rule out GD.

During the COVID-19 pandemic, many maternity services temporarily used alternatives to the GTT, including HbA1c. UK research subsequently showed that these approaches could miss a substantial number of women with gestational diabetes. In one study, the temporary screening pathway failed to detect 57% of women subsequently identified as having GD.(15) In another, an HbA1c threshold of 39 mmol/mol at 28 weeks identified only 26% of women who met the NICE criteria for GD.(16)

NICE recommends measuring HbA1c when gestational diabetes is diagnosed to help identify women who may have had type 2 diabetes before pregnancy. This is a different use of HbA1c and does not mean that it can replace a GTT for diagnosing gestational diabetes.

What if my GTT result is borderline?

You may be told that your GTT result is “borderline” or that you have only narrowly missed the threshold for a gestational diabetes diagnosis.

Diagnostic thresholds have to draw a line somewhere, but the risks associated with higher blood sugar levels do not suddenly begin at that exact number. Research, including the HAPO study(9) discussed above, found that risks increased gradually as blood sugar levels increased.

For example, under NICE criteria, a fasting result of 5.5 mmol/L is below the diagnostic threshold of 5.6 mmol/L, but the difference between these two results is very small.

If your results are close to the diagnostic levels, particularly if you have other risk factors or concerns later in pregnancy, you can discuss with your maternity team whether further monitoring of your blood sugar levels would be appropriate.

For more information, see my guide to borderline gestational diabetes results.

Can I decline a GTT?

Yes. Like any medical test, having a GTT is your choice and you can decline it.

If you have concerns about having a GTT, discuss these with your maternity team. They should explain why the test has been recommended, the benefits and limitations of testing, and what alternatives may be available.

If you choose not to have a GTT, you may be offered home blood sugar monitoring instead, although this will depend on your individual circumstances and local maternity pathway.

Is the glucose drink used for a GTT safe?

The GTT involves drinking a solution containing 75 g of glucose. Understandably, some women are concerned about consuming this amount of glucose during pregnancy, particularly after reading claims online about the ingredients in glucose drinks.

The purpose of the test is to give the body a measured amount of glucose so that its response can be assessed. The glucose load is temporary and is specifically designed to identify problems with glucose regulation.

Different hospitals may use different glucose preparations. If you have allergies, intolerances or concerns about the ingredients in the drink your hospital uses, ask your maternity team what preparation is provided before your test.

There is no good evidence that the standard glucose drink used for a GTT causes harm to the pregnancy.(9)

What happens if my GTT is positive?

If one or more of your GTT results meet the diagnostic threshold used by your maternity service, you will be diagnosed with gestational diabetes.

You should be referred to your diabetes in pregnancy team and given information about monitoring your blood sugar levels, food and lifestyle changes, and any additional care or monitoring you may need during pregnancy.

Being diagnosed with gestational diabetes can feel overwhelming, particularly when you are given a lot of new information at once. My guide below takes you through what happens next and what you can do while waiting for your first diabetes appointment.

Just diagnosed with gestational diabetes?

Read my step-by-step guide: Gestational Diabetes Diagnosis: What Happens Next

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Gestational Diabetes Guidelines

NICE NG3 – Diabetes in pregnancy: management from preconception to the postnatal period (England, Wales and Northern Ireland)

SIGN 171 – Management of diabetes in pregnancy (Scotland)

HSE – Gestational diabetes (Republic of Ireland)