Glucose Test in Pregnancy: Timing and What to Expect

The glucose test in pregnancy is a key screening tool for gestational diabetes, helping to protect the health of both mother and baby. In the UK, the oral glucose tolerance test (OGTT) is typically offered between 24 and 28 weeks of gestation, in line with NICE guideline NG3. However, timing can vary depending on your individual risk factors. This article explains when the test is carried out, who is eligible on the NHS, what to expect on the day, how to interpret your results, and what side effects, if any, you might experience.

When do you have the glucose test during pregnancy?

The glucose tolerance test is typically carried out between 24 and 28 weeks of gestation, as this is when gestational diabetes most commonly becomes detectable. Women with previous gestational diabetes are offered earlier testing, from around 16 to 18 weeks.

The glucose tolerance test (GTT) in pregnancy is typically carried out between 24 and 28 weeks of gestation, in line with NICE guideline NG3 (Diabetes in pregnancy).^[1][5] This timing reflects the point at which gestational diabetes most commonly becomes detectable, as pregnancy hormones increasingly affect how the body processes insulin during the second trimester.

For women who have had gestational diabetes in a previous pregnancy, NICE NG3 recommends that testing is offered as early as possible after the booking appointment, rather than waiting until 24 weeks. If that early result is normal, a repeat test is offered at 24 to 28 weeks. Some local services schedule this early test at around 16 to 18 weeks, though the exact timing may vary by integrated care board (ICB) or trust protocol.

The test is arranged through your midwife or GP and takes place at a hospital or community clinic. You will usually receive a referral letter with instructions on how to prepare. It is important to attend the appointment as scheduled, since gestational diabetes that goes undetected can increase the risk of complications for both mother and baby, including a larger-than-average baby (macrosomia), premature birth, and pre-eclampsia, as noted by NHS guidance on gestational diabetes.

If you develop symptoms that could suggest high blood sugar at any point in pregnancy, such as increased thirst, frequent urination, or unusual fatigue, you should contact your midwife or GP promptly rather than waiting for a routine appointment. Early identification allows for timely management and monitoring throughout the remainder of the pregnancy.

Who is offered a glucose tolerance test on the NHS?

The NHS offers the GTT to women with at least one risk factor, such as BMI above 30, a previous baby weighing 4.5 kg or more, gestational diabetes in a prior pregnancy, or a first-degree relative with diabetes.

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, so understanding whether you fall into a higher-risk group is important.

NICE NG3 identifies the following as key risk factors that should prompt a GTT:

• A body mass index (BMI) above 30 kg/m² at the start of pregnancy

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

• Gestational diabetes in a previous pregnancy

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

• A family origin associated with a higher prevalence of type 2 diabetes, including South Asian, Black African or Caribbean, and Middle Eastern backgrounds^[1]

Some NHS trusts and ICBs also consider additional factors, such as polycystic ovary syndrome (PCOS) or a history of unexplained stillbirth, as grounds for offering a GTT. These are not listed as national screening criteria in NICE NG3, so whether they apply to you will depend on your local service's protocol.

If you are unsure whether you meet the criteria, your community midwife is the right person to ask at your booking appointment or at any subsequent antenatal visit. It is worth raising any family history of diabetes or concerns about your weight at that early stage, so that appropriate referrals can be made in good time. Women who do not have identifiable risk factors are not routinely offered the GTT on the NHS, though they may choose to access testing privately.

What happens during the test and how should you prepare?

The OGTT requires an overnight fast, followed by a fasting blood draw, a 75 g glucose drink, and a second blood sample two hours later. The full appointment lasts approximately two to two and a half hours.

The oral glucose tolerance test (OGTT) involves fasting overnight, drinking a glucose solution, and having blood drawn at set intervals to measure how your body processes sugar. This procedure is described in NICE guideline NG3 and in NHS patient information on gestational diabetes.

On the morning of the test, you will be asked to have nothing to eat or drink (other than water) from midnight, or for at least eight to ten hours beforehand. You should also avoid strenuous exercise on the day of the test, as this can affect results.

When you arrive at the clinic, a healthcare professional will take a fasting blood sample from a vein in your arm. You will then be given a measured glucose drink containing 75 g of glucose dissolved in water. The drink is sweet and some women find it unpleasant, but it must be consumed within a set time, usually five minutes. You will then need to remain at the clinic and rest quietly for two hours, during which you should not eat, drink anything other than water, or smoke.

A second blood sample is taken two hours after the glucose drink. Some clinics also take an intermediate sample at one hour, though this varies by local protocol. If you vomit after drinking the glucose solution, the test may need to be rescheduled; inform the clinic staff straight away if this happens. The entire appointment usually lasts around two to two and a half hours, so it is worth planning your day accordingly and bringing something to read or listen to.

Practical tips for the day include:

• Wearing comfortable clothing with easy access to the inner arm

• Bringing water to sip during the waiting period

• Arranging childcare if needed, as you will need to stay at the clinic throughout

• Informing the clinic of any medications you take regularly, as some can affect glucose readings

What do your results mean and what happens next?

Gestational diabetes is diagnosed if fasting plasma glucose is 5.6 mmol/litre or above, or the two-hour level is 7.8 mmol/litre or above. A positive result leads to referral to a specialist joint antenatal and diabetes clinic within one week.

A normal glucose tolerance test result means that your blood sugar levels are within the expected range and gestational diabetes has not been identified at this point in your pregnancy. According to NICE guideline NG3, gestational diabetes is diagnosed if the fasting plasma glucose level is 5.6 mmol/litre or above, or if the two-hour plasma glucose level is 7.8 mmol/litre or above. Your results will be interpreted by a clinician in the context of your full clinical picture.

If your results are within the normal range, no further GTT is usually required unless new risk factors emerge or symptoms develop later in pregnancy. Your routine antenatal care will continue as planned.

If gestational diabetes is diagnosed, NICE NG3 recommends that you are referred to a specialist joint antenatal and diabetes clinic, typically within one week of receiving your results.^[1][6] Management usually begins with dietary advice and blood glucose monitoring at home. A structured eating plan, focusing on reducing refined carbohydrates and increasing fibre, is the first-line approach. Regular physical activity, where safe and appropriate, is also encouraged.

If blood glucose levels remain above target despite dietary changes, medication may be considered. Metformin is often the first oral option, and insulin therapy may be introduced if targets are still not met. Your care team will guide you through these steps and monitor both your health and your baby's growth more closely throughout the remainder of the pregnancy.

After birth, gestational diabetes usually resolves. However, women who have had it carry a higher lifetime risk of developing type 2 diabetes.^[3][8] NICE NG3 recommends a fasting plasma glucose test between 6 and 13 weeks postpartum to check that blood sugar has returned to normal.^[3][4] If this test is not done within that window, an HbA1c test can be offered from 13 weeks. Longer-term, annual HbA1c testing is recommended to monitor ongoing diabetes risk.

Are there any risks or side effects from the glucose test?

The GTT is safe and serious complications are very uncommon. Mild side effects such as nausea, dizziness, and bruising at the blood draw site are the most frequently reported and usually resolve quickly.

The glucose tolerance test is a safe and well-established procedure, and serious complications are very uncommon. The most frequently reported side effects relate to the glucose drink itself and the fasting period required beforehand.

Common, mild side effects include:

• Nausea or feeling sick after drinking the glucose solution

• Lightheadedness or dizziness, particularly if you are sensitive to fasting

• Headache during the waiting period

• Discomfort or minor bruising at the blood draw site

These effects are generally short-lived and resolve once the test is complete and you are able to eat and drink normally. If you feel faint or unwell during the test, inform the clinic staff immediately so they can provide appropriate support.

The blood draw itself carries the same small risks as any venepuncture, including temporary soreness, bruising, or, very rarely, a small haematoma (collection of blood under the skin). Infection at the needle site is extremely rare when standard clinical hygiene is followed.

The 75 g oral glucose load used in the standard GTT is considered safe in pregnancy when administered under clinical supervision, and is the dose specified in NICE guideline NG3.^[1][7]

If you experience any unexpected symptoms after the test, such as prolonged nausea, vomiting, or feeling unwell for several hours, contact your midwife or GP for advice. For urgent concerns outside of normal hours, NHS 111 can provide guidance. If you experience a suspected side effect from a medicine or medical device, this can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk, which accepts reports from both patients and healthcare professionals.

Scientific References

1. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Recommendations.
2. Diabetes in pregnancy (NG3) – Your blood glucose levels during pregnancy.
3. Diabetes in pregnancy (NG3) – Follow-up care and future pregnancy.
4. Quality statement 4: Postnatal testing and referral (QS109).
5. Gestational diabetes.
6. Gestational diabetes – Treatment.
7. Diabetes – Treatment summaries.
8. The life course perspective of gestational diabetes.
9. Comprehensive management of gestational diabetes mellitus: practical efficacy of exercise therapy and sustained intervention strategies.
10. Optimizing diabetes care in pregnancy: A systematic review of glycemic targets, treatment initiation, and glucose-lowering pharmacological management for T1DM, T2DM, and GDM.
11. Effectiveness of Metformin Prolonged-Release Formulation on Achievement of Optimal Glycemic Control in Gestational Diabetes Mellitus: Protocol for a Pilot, Randomized, Double-Blind, Clinical Trial.

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