Glucose Tolerance Test in Pregnancy Explained

The glucose tolerance test in pregnancy is a key diagnostic tool used to identify gestational diabetes mellitus, a condition that affects blood sugar regulation during pregnancy. Carried out as an oral glucose tolerance test (OGTT) using a 75 g glucose solution, it is offered on the NHS to women with recognised risk factors, typically between 24 and 28 weeks of pregnancy, in line with NICE guideline NG3. Because gestational diabetes rarely causes obvious symptoms, targeted screening is essential. Early detection allows healthcare teams to manage the condition promptly, reducing risks to both mother and baby.

What is the glucose tolerance test and why is it done in pregnancy?

The glucose tolerance test in pregnancy is a 75 g oral fasting test used to diagnose gestational diabetes, offered between 24 and 28 weeks to women at increased risk, as placental hormones can impair insulin effectiveness.

The glucose tolerance test (GTT) in pregnancy is a diagnostic blood test used to detect gestational diabetes mellitus (GDM), a condition in which blood sugar levels rise above normal during pregnancy. During pregnancy, hormones produced by the placenta can reduce the effectiveness of insulin, a process known as insulin resistance. For most women, the pancreas compensates by producing more insulin, but when it cannot keep pace, blood glucose rises to levels that can affect both mother and baby.

In UK practice, the diagnostic test is the oral glucose tolerance test (OGTT), a 75 g fasting test carried out in line with NICE guideline NG3 (Diabetes in pregnancy: management from preconception to the postnatal period). It involves measuring blood glucose before and after drinking a standard glucose solution, allowing clinicians to assess how efficiently the body processes sugar under a controlled challenge. For women identified as being at increased risk, the test is typically offered between 24 and 28 weeks of pregnancy, when placental hormones are at a level sufficient to reveal underlying glucose regulation problems (NICE NG3).

Gestational diabetes often causes no obvious symptoms, which is why targeted screening is important. Left undetected and unmanaged, it is associated with complications including a larger-than-average baby (macrosomia), increased likelihood of caesarean section, and a higher risk of the baby developing low blood sugar after birth. Early identification through the GTT allows healthcare teams to put appropriate management in place promptly, reducing these risks significantly.

Who is offered a glucose tolerance test on the NHS?

The NHS offers the OGTT to women with risk factors including BMI above 30, previous gestational diabetes, a previous baby weighing 4.5 kg or more, a first-degree relative with type 2 diabetes, or a high-risk family origin.

The NHS offers a glucose tolerance test to pregnant women who have one or more recognised risk factors for gestational diabetes, as set out in NICE guideline NG3. Not every pregnant woman is routinely screened; the test is targeted at those considered at higher risk based on clinical and personal history.

According to NICE NG3, the following risk factors trigger an offer of the OGTT:

• BMI above 30 kg/m² at the first antenatal appointment

• Previous gestational diabetes in an earlier pregnancy

• A previous large baby, typically defined as a birth weight of 4.5 kg or above

• A first-degree relative with type 2 diabetes (parent, sibling or child)

• Family origin associated with a higher prevalence of type 2 diabetes, including South Asian, Black African, Black African-Caribbean, and Middle Eastern backgrounds

Significant glycosuria detected at routine antenatal checks (for example, 2+ or more on one occasion, or 1+ on two or more occasions) can also prompt an earlier OGTT, and your midwife will advise you if this applies.^[1][2]

Women who had gestational diabetes in a previous pregnancy are offered an early OGTT, usually between 16 and 18 weeks, and a repeat test at 24 to 28 weeks if the first result is normal (NICE NG3). This earlier testing reflects the higher likelihood of recurrence in subsequent pregnancies.

If you are unsure whether you meet the criteria, your midwife or GP can review your history at your booking appointment and refer you for testing if appropriate. It is worth raising any family history of diabetes or concerns about your weight at that early stage, so that screening can be arranged in good time.

What happens during the test and how should you prepare?

You must fast for eight to ten hours before the test; a fasting blood sample is taken, you drink a 75 g glucose solution, then a second sample is taken two hours later, making the appointment around two to two and a half hours in total.

The oral glucose tolerance test requires you to fast for at least eight to ten hours beforehand, which means having nothing to eat or drink except water from the evening before your appointment. You should continue taking any prescribed medications unless your midwife or doctor advises otherwise, and you should avoid strenuous exercise on the morning of the test.

When you arrive at the clinic or hospital, a healthcare professional will take a fasting blood sample from a vein in your arm. You will then be asked to drink a glucose solution containing 75 g of glucose dissolved in water, in line with NICE NG3. The drink is sweet and some women find it causes mild nausea. After drinking it, you will need to remain at the clinic and rest quietly for two hours, avoiding food, drink (other than water), and significant physical activity, as these can affect the result.

A second blood sample is taken two hours after the glucose drink. These two samples, the fasting and the two-hour values, are the measurements used to make a diagnosis under NICE criteria. Some units also take a one-hour sample, though this is not universal across all NHS trusts and the one-hour value is not part of the NICE diagnostic thresholds.

If you vomit the glucose drink or feel too unwell to complete the test, let the clinic staff know straight away, as the test may need to be rescheduled to ensure an accurate result.

The whole appointment typically takes around two to two and a half hours. Bringing a book or something to occupy yourself during the waiting period is advisable. Results are usually communicated within a few days, either by your midwife, GP, or the antenatal team, depending on local arrangements.

What do your results mean for you and your baby?

Gestational diabetes is diagnosed if fasting glucose reaches 5.6 mmol/L or above, or two-hour glucose reaches 7.8 mmol/L or above; either threshold alone is sufficient for diagnosis under NICE NG3.

A glucose tolerance test result is considered diagnostic for gestational diabetes if either the fasting blood glucose level reaches 5.6 mmol/L or above, or the two-hour blood glucose level reaches 7.8 mmol/L or above, in line with NICE guideline NG3. Both thresholds are used independently, meaning that exceeding either one is sufficient for a diagnosis, even if the other value is within the normal range. Any one-hour value, if measured, is not used for diagnosis under NICE criteria.

If your results fall below both thresholds, gestational diabetes is not diagnosed at that point. However, if you develop symptoms such as increased thirst, frequent urination, or fatigue later in pregnancy, it is worth discussing these with your midwife, as symptoms can occasionally emerge after a normal test result.

A positive result does not mean that something has gone wrong or that you have done anything to cause it. Gestational diabetes is a recognised complication of pregnancy influenced by hormonal and genetic factors. What matters most is that it has been identified, because prompt management substantially reduces the associated risks.

For your baby, uncontrolled gestational diabetes increases the risk of macrosomia, which can complicate delivery, as well as neonatal hypoglycaemia (low blood sugar in the newborn), premature birth, and, in rare cases, stillbirth. For you, it raises the likelihood of a caesarean section and pre-eclampsia, and it is associated with a higher long-term risk of developing type 2 diabetes after the pregnancy.^[14][12] Your antenatal team will explain what your specific result means and outline the next steps clearly.

How is gestational diabetes managed if the test comes back positive?

Management begins with dietary changes and blood glucose monitoring; if NICE targets are not met within one to two weeks, metformin is offered, followed by insulin if needed, alongside increased antenatal surveillance.

Gestational diabetes is managed through a combination of dietary changes, physical activity, blood glucose monitoring, and, where necessary, medication, all guided by a specialist multidisciplinary team in line with NICE guideline NG3. Following a positive result, you will typically be referred to a joint obstetric and diabetes clinic, where a dietitian, diabetes specialist nurse, obstetrician, and midwife work together to support you.

Dietary and lifestyle measures are the first line of management and are effective for many women:

• Reducing refined carbohydrates and sugary foods

• Eating regular, balanced meals with a lower glycaemic load

• Incorporating moderate physical activity, such as walking after meals, where safe to do so

• Self-monitoring blood glucose levels at home using a finger-prick device, usually before breakfast and one hour after meals

NICE NG3 sets the following capillary blood glucose targets for women with gestational diabetes: fasting glucose of 5.3 mmol/L or below; one-hour post-meal glucose of 7.8 mmol/L or below; and two-hour post-meal glucose of 6.4 mmol/L or below. If these targets are not met within one to two weeks of starting lifestyle measures, medication is considered.

Metformin is often the first pharmacological option, as it is well established in pregnancy and is taken orally.^[2][10] If metformin is insufficient or not tolerated, insulin therapy is introduced. Insulin does not cross the placenta and is safe for the baby.^[15][16] In some situations, insulin may be considered as the first-line medication from the outset, for example if fasting glucose is particularly elevated at diagnosis, or if there are signs of macrosomia or polyhydramnios on ultrasound scan.^[2][8] The type and dose of insulin are adjusted throughout pregnancy as requirements change.

You will have more frequent antenatal appointments and additional ultrasound scans to monitor your baby's growth. After delivery, gestational diabetes usually resolves, but NICE NG3 recommends a fasting blood glucose test at six to thirteen weeks postnatally, and annual HbA1c checks thereafter, given the increased lifetime risk of type 2 diabetes.^[4] Breastfeeding is encouraged, as it supports blood glucose regulation in both mother and baby.

Scientific References

1. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Recommendations.
2. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Recommendations (chapter 1).
3. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Overview.
4. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Follow-up care and future pregnancy.
5. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Full guideline.
6. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Ethnic health inequalities.
7. Gestational diabetes.
8. Gestational diabetes – Treatment.
9. Diabetes | Treatment summaries – BNF – NICE.
10. Diabetes, pregnancy and breast-feeding | Treatment summaries – BNF – NICE.
11. Metformin hydrochloride | Drugs – BNF – NICE.
12. A Systematic Review to Compare Adverse Pregnancy Outcomes in Women with Pregestational Diabetes and Gestational Diabetes.
13. Gestational diabetes and stillbirth: a systematic review and meta-analysis.
14. Comprehensive management of gestational diabetes mellitus: practical efficacy of exercise therapy and sustained intervention strategies.
15. Transplacental Passage of Insulin in Pregnant Women with Insulin-Dependent Diabetes Mellitus.
16. The care of pregestational and gestational diabetes and drug metabolism considerations.
17. Metformin safety during pregnancy in women with gestational diabetes mellitus: A systematic review and meta-analysis of maternal, neonatal and long-term outcomes.

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