The 2 hour glucose tolerance test in pregnancy is the standard NHS diagnostic test for gestational diabetes mellitus (GDM). Offered at 24–28 weeks to women with identified risk factors, it measures how effectively your body processes glucose by comparing a fasting blood sample with one taken two hours after drinking a 75 g glucose solution. Knowing what constitutes a normal result — and what an abnormal one may mean for you and your baby — can help you feel informed and prepared. This article explains the NICE-recommended thresholds, what your results indicate, and the next steps if gestational diabetes is diagnosed.

What Is the 2 Hour Glucose Tolerance Test in Pregnancy?

The 2 hour glucose tolerance test (OGTT) in pregnancy is a diagnostic blood test for gestational diabetes, involving a fasting sample and a second sample two hours after drinking 75 g of glucose, offered at 24–28 weeks to women with identified risk factors per NICE NG3.

The 2 hour glucose tolerance test (GTT) in pregnancy — formally known as the oral glucose tolerance test (OGTT) — is a diagnostic blood test used to detect gestational diabetes mellitus (GDM). Gestational diabetes is a condition in which blood glucose levels rise above normal during pregnancy, typically because the body cannot produce sufficient insulin to meet the increased demands placed upon it. It is one of the more common complications of pregnancy in the UK, though prevalence estimates vary depending on the population studied and the testing strategy used.

According to NICE guideline NG3 (Diabetes in pregnancy), the OGTT is offered at 24–28 weeks of pregnancy to women with identified risk factors. For women who have had gestational diabetes in a previous pregnancy, testing should be arranged as soon as possible after booking, with a repeat OGTT at 24–28 weeks if the initial result is normal. You are likely to be offered an OGTT if you have one or more of the following risk factors:

• A BMI above 30

• A previous baby weighing 4.5 kg (approximately 10 lb) or more

• A personal history of gestational diabetes in a previous pregnancy

• A first-degree relative with type 2 diabetes

• A family origin associated with a higher prevalence of type 2 diabetes (for example, South Asian, Black Caribbean, or Middle Eastern)

In addition, persistent glycosuria — glucose detected in the urine (for example, 2+ on one occasion, or 1+ on two or more occasions) — may also prompt your midwife or GP to arrange diagnostic testing.

The procedure itself is straightforward. You will be asked to fast overnight (usually for at least 8–10 hours, taking water only, and avoiding smoking or strenuous exercise) before attending your appointment. A fasting blood sample is taken first, then you drink a glucose solution containing 75 g of glucose. A second blood sample is drawn exactly two hours later. You will be asked to remain on site between the two samples. These two readings together allow clinicians to assess how effectively your body processes glucose, providing a clear picture of your metabolic function during pregnancy.

(NICE NG3; NHS Gestational diabetes)

NHS Normal Range for the Oral Glucose Tolerance Test

Under NICE NG3, a normal OGTT result is a fasting plasma glucose below 5.6 mmol/L and a 2 hour plasma glucose below 7.8 mmol/L; meeting or exceeding either threshold alone is sufficient to diagnose gestational diabetes.

Understanding your OGTT results requires knowing the thresholds used by the NHS, which are set out in NICE guideline NG3. The test produces two key measurements: a fasting venous plasma glucose level and a 2 hour venous plasma glucose level. Both values are expressed in millimoles per litre (mmol/L).

According to NICE, gestational diabetes is diagnosed if either of the following thresholds is met or exceeded:

• Fasting plasma glucose: 5.6 mmol/L or above

• 2 hour plasma glucose: 7.8 mmol/L or above

These criteria are dichotomous: if either value meets or exceeds its respective threshold, a diagnosis of gestational diabetes is confirmed. Meeting one threshold alone is sufficient — both do not need to be elevated.

If your results fall below both of these thresholds, your OGTT is considered normal and gestational diabetes is not diagnosed at that point in your pregnancy. A normal fasting result is therefore below 5.6 mmol/L, and a normal 2 hour result is below 7.8 mmol/L.

It is worth noting that these thresholds differ from those used in some other countries, such as the United States, where the American Diabetes Association applies different cut-off values. In the UK, the NICE thresholds are the standard used across NHS maternity services, so it is important to interpret your results in the context of UK guidance rather than international comparisons you may encounter online.

If your results do not meet the diagnostic thresholds but your healthcare team has other clinical concerns — for example, ongoing symptoms or additional risk factors — your midwife or obstetrician may recommend closer monitoring of your blood glucose levels for the remainder of your pregnancy. Always discuss your individual results with your healthcare team, as clinical context matters alongside the numbers.

(NICE NG3; NHS Gestational diabetes — diagnosis)

What Abnormal Results May Mean for You and Your Baby

Abnormal OGTT results confirming gestational diabetes increase risks of macrosomia, neonatal hypoglycaemia, and shoulder dystocia for the baby, and pre-eclampsia or future type 2 diabetes for the mother, though the condition is manageable with appropriate care.

If your OGTT results meet or exceed the NICE diagnostic thresholds — whether it is the fasting value, the 2 hour value, or both — you will be diagnosed with gestational diabetes. Receiving this diagnosis can feel worrying, but it is important to understand that gestational diabetes is manageable, and with appropriate care, most women go on to have healthy pregnancies and babies.

Unmanaged or poorly controlled gestational diabetes can, however, carry risks. For the baby, elevated maternal blood glucose crosses the placenta, causing the baby's pancreas to produce extra insulin. This can lead to:

• Macrosomia (a larger-than-average baby), which may complicate delivery

• Shoulder dystocia (difficulty delivering the baby's shoulders during birth)

• Neonatal hypoglycaemia (low blood sugar in the newborn after birth)

• Polyhydramnios (excess amniotic fluid)

• Increased risk of preterm birth

• Higher likelihood of the baby developing obesity or type 2 diabetes later in life

For the mother, gestational diabetes increases the risk of:

• Pre-eclampsia (high blood pressure in pregnancy)

• The need for induction of labour or caesarean section

• Developing type 2 diabetes in the years following pregnancy

Gestational diabetes is often entirely asymptomatic, which is precisely why screening is offered routinely to those with risk factors rather than relying on symptoms alone to prompt investigation.

(NICE NG3; NHS Gestational diabetes — complications)

Next Steps After Your Glucose Tolerance Test Results

A normal result means routine antenatal care continues; a gestational diabetes diagnosis prompts referral to a joint diabetes and antenatal clinic, with management via dietary changes, blood glucose monitoring, and medication such as metformin or insulin if targets are not met.

If your OGTT results are normal, you will typically continue with your routine antenatal care. Your midwife may advise you to maintain a balanced diet and stay physically active, as these measures support healthy blood glucose levels throughout pregnancy regardless of your GTT outcome. If risk factors remain present, a repeat OGTT may be offered later in pregnancy.

If you are diagnosed with gestational diabetes, NICE recommends that you are referred promptly to a joint diabetes and antenatal clinic. Your care team will usually include a midwife, obstetrician, diabetes specialist nurse, and dietitian. Initial management typically focuses on:

• Dietary modification: Reducing refined carbohydrates and sugars, increasing fibre, and spacing meals evenly throughout the day

• Physical activity: Regular moderate exercise, such as walking, can meaningfully improve insulin sensitivity

• Blood glucose self-monitoring: You will be provided with a glucometer and advised on target capillary blood glucose levels. NICE recommends the following targets for women with gestational diabetes:

• Fasting: 5.3 mmol/L or below
• 1 hour after meals: 7.8 mmol/L or below
• (Or, if 2 hour post-meal monitoring is used: 6.4 mmol/L or below)

If blood glucose levels remain above target despite lifestyle changes, medication may be introduced. Metformin is often considered as a first-line pharmacological option alongside dietary changes, as recommended by NICE NG3. It works by reducing hepatic glucose production and improving insulin sensitivity. It is important to note that while metformin is widely used for gestational diabetes across NHS services in line with NICE guidance, it may be classified as off-label use according to some individual product SmPCs (Summaries of Product Characteristics); it should therefore be prescribed and supervised by a specialist.

Insulin therapy may be recommended instead of, or in addition to, metformin in certain circumstances. NICE advises that insulin should be offered immediately (rather than trialling metformin first) if the fasting plasma glucose at diagnosis is 7.0 mmol/L or above. Insulin should also be considered if fasting glucose is between 6.0 and 6.9 mmol/L and there are concerns about fetal growth (such as macrosomia or polyhydramnios). Both metformin and insulin are considered safe for use in pregnancy under medical supervision.

After delivery, gestational diabetes usually resolves, but it carries longer-term implications. NICE recommends that women who have had gestational diabetes are offered a fasting plasma glucose test at the 6–13 week postnatal check to exclude type 2 diabetes. If this test is not completed within that window, an HbA1c measurement can be offered from 13 weeks postpartum onwards. Annual HbA1c testing is then recommended thereafter, given the elevated lifetime risk of developing type 2 diabetes.

If at any point during or after pregnancy you experience symptoms such as excessive thirst, frequent urination, or unexplained fatigue, contact your GP promptly for assessment.

(NICE NG3; NHS Gestational diabetes — treatment; BNF / EMC SmPCs for metformin and insulin)

Frequently Asked Questions