The glucose tolerance test is the one antenatal appointment people arrive at annoyed. You have fasted overnight, you are two hours from breakfast, and you are being asked to drink something intensely sweet at eight in the morning and then sit still.
It is also one of the few tests in pregnancy that looks for a condition with no symptoms at all. Gestational diabetes does not make you feel unwell; it is found by testing or not found. This guide covers who is offered it, exactly how the morning goes, how to prepare so the result means something, and what happens afterwards.
The short answer: the oral glucose tolerance test is offered between 24 and 28 weeks to anyone with a risk factor for gestational diabetes. You fast for 8 to 10 hours, have a blood sample taken, drink a measured glucose solution, sit still for two hours and have a second sample. It takes about two hours in total. Gestational diabetes causes no symptoms, which is the entire reason the test exists.
Who is offered it
The UK screens by risk factor rather than universally. Any one of these is enough to be offered the test:
🩸 The risk factors
- Age over 40
- A BMI above 30
- A previous baby weighing 4.5 kg (10 lb) or more
- Gestational diabetes in a previous pregnancy — tested much earlier, and again at 24–28 weeks
- A parent or sibling with diabetes
- A South Asian, Black, African-Caribbean or Middle Eastern family origin
- Previous gastric bypass or weight-loss surgery
Many countries test every pregnancy instead. Universal testing finds more cases; risk-based testing puts fewer women through the morning described below. Both are defensible, which is why the two approaches coexist.
How the morning actually goes
- You arrive fasting. Nothing to eat or drink for 8 to 10 hours except water — and check locally, because units differ on whether water is unrestricted.
- First blood sample. This is your fasting glucose, and it is often the value that decides the result.
- The drink. A measured glucose solution, drunk within a few minutes. It is very sweet and not carbonated.
- Two hours of sitting. No walking about, no coffee, no smoking. Physical activity lowers blood glucose and would make the test measure your morning rather than your metabolism.
- Second blood sample. Then you can eat — and bringing food is the single most useful thing you can do for yourself.
Pack for two hours, not for ten minutes. Breakfast for afterwards, water, something to read, and a jumper — you will be sitting still in a hospital waiting area for longer than you think. Book the earliest slot available so the overnight fast covers your sleep instead of your morning.
Preparing so the result means something
The overnight fast is the rule everyone is told. The one nobody mentions matters just as much: eat normally in the days beforehand.
Cutting carbohydrates in the week before a glucose test feels like sensible preparation and works in the opposite direction. A body that has been low on carbohydrate handles a sudden large glucose load worse, not better, which can push a result upwards. The test is designed to measure how you normally process glucose, so the useful preparation is to be normal.
- Do not diet beforehand. Eat your usual meals, including your usual carbohydrates.
- Do not fast longer than asked. Fourteen hours is not more rigorous than ten; it is a different test.
- Mention anything relevant. Steroids, some antipsychotics and beta-blockers all affect glucose handling, as does a recent illness.
- Say if you have had weight-loss surgery. The standard OGTT can produce misleading results after a gastric bypass, and an alternative is usually arranged.
Why a result can be misleading
Two things invalidate the test outright, and both are common enough to be worth knowing.
Vomiting the drink. If it does not stay down, the test cannot be interpreted and has to be repeated on another day. Say so rather than sitting through the two hours regardless.
Not staying still. Walking to another department and back between samples lowers the second value. It is the most common reason a test underestimates.
Beyond those, a single abnormal value on a single morning is exactly what the diagnosis rests on, which is why hospitals repeat borderline results in some circumstances rather than treating them as final. Asking what your actual numbers were, and what threshold your unit applies, is the most useful question you can ask about this test.
The thresholds are not the same everywhere. The UK uses NICE criteria; much of the rest of the world uses WHO criteria, which are lower and diagnose more women from the same blood sample. Comparing results with someone in another country will produce confusion rather than reassurance. Our gestational diabetes guide sets the two side by side.
If the result is positive
You will be referred to a joint diabetes and antenatal clinic, usually within a week, and given a glucose meter to test at home. Most women diagnosed with gestational diabetes control it through diet and monitoring alone; some need metformin, and some need insulin. None of that is a failure of anything you did.
The rest of the pregnancy changes in specific ways: growth scans in the third trimester, a conversation about timing of birth, and monitoring of the baby's blood sugar in the first day after delivery. Our gestational diabetes diet guide covers the eating side in detail, and the fetal growth percentiles guide explains the scans that follow.
Gestational diabetes usually resolves after birth, and a follow-up glucose test is arranged 6 to 13 weeks later. It also raises your long-term risk of type 2 diabetes, which is why that follow-up test — the one that is easiest to skip with a newborn — is the one worth keeping.
When to call your midwife
Call the same day if: after a diagnosis of gestational diabetes you have blood sugar readings that are persistently high or that drop very low with shakiness and confusion, or at any point in pregnancy you have reduced or changed fetal movements after 24 weeks, severe headache with visual disturbance, sudden swelling of the face and hands, or a fever.
On the day of the test itself, tell staff if you feel faint rather than trying to sit it out — that is common, expected and easily managed, and it is not a reason for anyone to think less of you.
Frequently Asked Questions
When is the glucose tolerance test done?
Between 24 and 28 weeks. That window is when placental hormones peak and insulin resistance is at its highest, which is when gestational diabetes becomes detectable. If you have had gestational diabetes in a previous pregnancy, testing is usually offered much earlier — soon after the booking appointment — and repeated at 24 to 28 weeks if the first result was normal.
Who is offered the test?
In the UK it is offered on risk factors rather than to everyone: age over 40, a BMI above 30, a previous baby weighing 4.5 kg or more, previous gestational diabetes, a parent or sibling with diabetes, a South Asian, Black, African-Caribbean or Middle Eastern family origin, or previous weight-loss surgery. Many other countries test every pregnancy. Neither approach is wrong; they trade off detection against the number of women tested.
How long does the test take?
About two hours, most of it waiting. You arrive fasting, have a blood sample taken, drink a measured glucose solution, then sit still for two hours before a second sample. Sitting still is part of the test rather than a courtesy — walking around lowers blood glucose and can change the result.
What can I eat or drink before the test?
Nothing for 8 to 10 hours beforehand except water, and check with your hospital, because some units allow water freely and others do not. What matters as much is the days before: eating unusually low-carbohydrate meals in the week leading up to it can make your body handle the glucose load worse, not better, so eat normally rather than "healthily".
Can I refuse the glucose tolerance test?
Yes, like every antenatal test. It is worth knowing what you would be declining: untreated gestational diabetes raises the chance of a large baby, birth complications and neonatal low blood sugar, and it usually produces no symptoms at all, so there is nothing to notice instead. Some units will offer alternatives such as HbA1c or home glucose monitoring, which are less accurate but better than nothing.
Why do I feel sick after the glucose drink?
Because it is a large, fast dose of sugar on an empty stomach, and in pregnancy that lands harder. Nausea, sweating, light-headedness and a headache are all common and none of them mean anything is wrong. Tell the staff if you vomit — the test is invalid if the drink does not stay down, and it will have to be rebooked rather than interpreted.
What if my result is only slightly over?
A borderline result is still a diagnosis of gestational diabetes, but the management follows the numbers rather than the label. Most women diagnosed on a marginal result control their glucose with diet and monitoring alone and never need medication. Ask what your actual values were and what the threshold used at your hospital is — those two numbers tell you far more than the word "positive".
Do the diagnostic thresholds differ between countries?
Yes, substantially, and the same blood sample can be a diagnosis in one country and normal in another. The UK uses NICE criteria; much of the rest of the world uses WHO criteria, which are lower and therefore diagnose more women. It is one of the most confusing things about this test if you compare notes internationally. Go by the criteria your own clinician uses.
The glucose tolerance test — at a glance
The OGTT is offered between 24 and 28 weeks to anyone with a risk factor for gestational diabetes: age over 40, BMI over 30, a previous baby of 4.5 kg or more, previous gestational diabetes, a first-degree relative with diabetes, a higher-risk family origin, or previous weight-loss surgery. You fast for 8 to 10 hours, give a blood sample, drink a measured glucose solution, sit still for two hours and give a second sample. Sitting still and keeping the drink down are both part of the test rather than etiquette. Eat normally in the days beforehand — cutting carbohydrates makes the result worse, not better. Diagnostic thresholds differ between countries, so ask for your actual numbers and the criteria your hospital uses. Gestational diabetes produces no symptoms, which is why a test exists at all.
Inside Baby Novum: the lab results journal stores each result with its date, gestational week and a photo of the report, and the weight tracker plots your gain against a WHO corridor derived from your pre-pregnancy BMI — so a borderline glucose result is read alongside your own trend instead of on its own. The calendar can hold the 6-to-13-week postnatal follow-up test, which is the one most easily forgotten.