Gestational diabetes test: who is tested, when, how
A gestational diabetes test checks how your body handles sugar (glucose) in pregnancy. In the US it is often called the glucose test in pregnancy, and the US and the UK decide who is tested, and with which test, in different ways.
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Medically reviewed by Dr Mary Davidson, Reproductive endocrinologist, checked on
This page describes how testing is organized and what the published guidance says. It is not a way to read your own result: the clinic that ordered your test interprets it, alongside everything else it knows about your pregnancy.
What gestational diabetes is
The NHS describes gestational diabetes as high blood sugar that develops during pregnancy and usually disappears after giving birth, and says it happens when the body cannot produce enough insulin to meet the extra needs of pregnancy (NHS, "Gestational diabetes", 2022). The CDC says it usually develops around the 24th week and may not cause symptoms, which is why testing between 24 and 28 weeks matters; it affects 5 to 9 in 100 pregnancies in the US each year (CDC, "Gestational Diabetes", 2024). How gestational diabetes is treated is a matter for your care team, and this page does not cover it.
Who is offered a gestational diabetes test
Gestational diabetes screening is where the US and the UK differ most. The US tests everyone; the UK tests people with a risk factor.
In the US: screening for everyone at 24 weeks or later
The US Preventive Services Task Force recommends screening for gestational diabetes in all pregnant people without symptoms at 24 weeks of pregnancy or after, a grade B recommendation. It found the evidence insufficient to recommend for or against screening before 24 weeks, and says screening typically happens before 28 weeks in the US, or later for people who start prenatal care after that (USPSTF, "Screening for Gestational Diabetes", 2021). ACOG says all pregnant women should be screened, that people with risk factors are tested early in pregnancy, and that everyone else is tested between 24 and 28 weeks (ACOG, "Gestational Diabetes", 2020).
The CDC lists these gestational diabetes risk factors (CDC, 2024):
- gestational diabetes in a previous pregnancy
- a previous baby who weighed over 9 pounds
- being overweight
- a family history of type 2 diabetes
- polycystic ovary syndrome (PCOS)
- being African American, Hispanic or Latino, American Indian, Alaska Native, Native Hawaiian or Pacific Islander
In the UK: a risk check at booking, and a test for those with a risk factor
NICE guideline NG3 asks midwives to assess the risk at the booking appointment and to offer testing to anyone with one of these risk factors: a body mass index (BMI) of 30 or over, a previous baby weighing 4.5 kg or more, previous gestational diabetes, a parent, brother, sister or child with diabetes, or an ethnicity with a high prevalence of diabetes (NICE, "Diabetes in pregnancy: management from preconception to the postnatal period (NG3)", 2015). The NHS adds being over 40 and having had a gastric bypass or other weight-loss surgery, and names south Asian, Black, African-Caribbean and Middle Eastern origin (NHS, "Gestational diabetes", 2022). The booking appointment is at around week 8 to 12.
NICE also says not to use the glucose challenge test, fasting glucose, random glucose, HbA1c or a urine test for glucose to assess the risk (NICE, 2015). So the 1-hour test that most US pregnancies start with is not part of the UK pathway at all.
| US | UK | |
|---|---|---|
| Who is tested | Everyone, at 24 weeks or later | Those with one or more risk factors at booking |
| Usual first test | 1-hour glucose challenge test (two-step), or a glucose tolerance test straight away (one-step) | 75 g 2-hour oral glucose tolerance test |
| Usual timing | 24 to 28 weeks; earlier with risk factors | 24 to 28 weeks; soon after booking if gestational diabetes in a previous pregnancy |
| Guidance | USPSTF 2021; ACOG | NICE NG3; NHS |
The BMI in these lists is the weight before or early in pregnancy. The pregnancy weight gain calculator shows how weight gain ranges are set from it, and preconception health covers what US and UK guidance says about weight before pregnancy.
When the test is done
When is the glucose test in pregnancy? In most pregnancies, between 24 and 28 weeks. The CDC says you will probably be tested between 24 and 28 weeks, and earlier if you are at higher risk (CDC, 2024). NIDDK gives the same window and says someone with an increased chance may be tested at the first prenatal visit (NIDDK, "Tests & Diagnosis for Gestational Diabetes", 2017). MedlinePlus says most pregnant women have the screening test between 24 and 28 weeks, and that it may be done earlier after a high urine glucose result at a routine visit or with a high risk of diabetes (MedlinePlus, "Glucose screening tests during pregnancy", 2026).
In the UK, the NHS says the test is done between 24 and 28 weeks, and that after gestational diabetes in a previous pregnancy you are offered a test soon after the booking appointment and again at 24 to 28 weeks if the first is normal (NHS, 2022). NICE gives the same rule (NICE, 2015).
So the gestational diabetes test week is usually somewhere from week 24 to week 28. 24 weeks pregnant sets out where the windows in different documents fall, and the guide to the second trimester places the test among the other checks of those months.
How the glucose test in pregnancy works
There are two kinds of glucose test during pregnancy: a short screening test, and a longer diagnostic test. The US uses either a two-step approach, which starts with the screening test, or a one-step approach that goes straight to the diagnostic test (USPSTF, 2021).
The 1-hour glucose challenge test
The glucose challenge test, also called the glucose screening test, is the first step of the US two-step approach. You drink a sweet liquid containing glucose and blood is drawn 1 hour later. NIDDK says you do not need to fast for it (NIDDK, 2017), and MedlinePlus says you do not need to prepare or change your diet (MedlinePlus, 2026). This is the 1 hour glucose test in pregnancy that many US patients are told about.
The glucose tolerance test
The oral glucose tolerance test (OGTT) measures blood glucose after a fast, then at set times after a glucose drink. Its length and dose differ:
- The 3-hour test (US two-step). MedlinePlus describes nothing to eat or drink except sips of water for 8 to 14 hours, a drink containing 100 g of glucose, and blood drawn before the drink and every 60 minutes for 3 hours. This is the 3 hour glucose test in pregnancy that follows a high screening result (MedlinePlus, 2026).
- The 2-hour test (US one-step). The same fast, a 75 g glucose drink, and blood drawn before the drink and twice more, an hour apart (MedlinePlus, 2026).
- The 2-hour test (UK). The NHS describes a blood test in the morning after 8 to 10 hours without food or drink, though water is usually allowed, then a glucose drink, 2 hours' rest and a second blood test; it takes about 2 hours (NHS, 2022). NICE specifies the 75 g 2-hour test (NICE, 2015).
| Test, and where it is used | Fasting first | Drink and blood draws |
|---|---|---|
| 1-hour glucose challenge test: US, step one of two | No | Glucose drink; 1 draw after 1 hour |
| 3-hour glucose tolerance test: US, step two | 8 to 14 hours | 100 g; 4 draws over at least 3 hours |
| 2-hour glucose tolerance test: US, one-step | 8 to 14 hours | 75 g; 3 draws over at least 2 hours |
| 2-hour glucose tolerance test: UK | 8 to 10 hours | 75 g; 2 draws over about 2 hours |
MedlinePlus says most women have no side effects. Drinking the solution is like drinking a very sweet soda, and some feel nauseated, sweaty or lightheaded afterwards; serious side effects are very uncommon (MedlinePlus, 2026).
How to prepare for the glucose test in pregnancy
What to eat before the glucose test in pregnancy depends on which test it is:
- For either US approach, MedlinePlus advises eating your normal food in the days before the test, and asking your provider whether any medicines you take can affect the result (MedlinePlus, 2026).
- For the 1-hour challenge test, no fast is needed (NIDDK, 2017).
- For a glucose tolerance test, you fast beforehand, as set out above, and you cannot eat during the test (MedlinePlus, 2026; NHS, 2022).
Some people look for ways to pass the test. The test measures how your body handles a set amount of glucose, and preparation instructions exist so the result reflects that. How to prepare for the gestational diabetes test is decided by the clinic running it, and its instructions override anything on this page.
What happens after a high result
A high result on the screening test is not a diagnosis. In the US two-step approach it leads to the longer glucose tolerance test, and MedlinePlus says about 2 out of 3 women who take that test do not have gestational diabetes (MedlinePlus, 2026). NIDDK says high values at any two or more of the blood test times mean gestational diabetes, and that your health care team will explain your results (NIDDK, 2017). A failed glucose test in pregnancy, in other words, usually means another test rather than a diagnosis.
Gestational diabetes test results are judged against fixed thresholds that the guidance publishes. They are printed here as each source gives them so that you can see what the rule is. They are not for checking a result of your own: laboratories and clinics may apply other criteria, and the meaning of a result is for the team that ordered it.
| Source and test | Values the source gives | Diagnosis when |
|---|---|---|
| NICE NG3, UK: 75 g 2-hour test | Fasting 5.6 mmol/L or above; 2 hours 7.8 mmol/L or above | Either value is reached |
| MedlinePlus, US: 100 g 3-hour test | Fasting 95 mg/dL (5.3 mmol/L) or above; 1 hour 180 mg/dL (10.0 mmol/L) or above; 2 hours 155 mg/dL (8.6 mmol/L) or above; 3 hours 140 mg/dL (7.8 mmol/L) or above | More than one value is high |
| MedlinePlus, US: 75 g 2-hour test | Fasting 92 mg/dL (5.1 mmol/L) or above; 1 hour 180 mg/dL (10.0 mmol/L) or above; 2 hours 153 mg/dL (8.5 mmol/L) or above | See the note below |
NICE diagnoses gestational diabetes at a fasting plasma glucose of 5.6 mmol/L or above or a 2-hour level of 7.8 mmol/L or above (NICE, 2015). MedlinePlus lists the US values as abnormal results; for the one-step test it says a result below them means no gestational diabetes, and it adds that when more than one result is higher than normal, you have gestational diabetes (MedlinePlus, 2026). For the 1-hour screening test, MedlinePlus treats a result above 140 mg/dL (7.8 mmol/L) as the cue for the longer test, while NIDDK says 140 or more (MedlinePlus, 2026; NIDDK, 2017). The two sources disagree at exactly 140, which is one reason a clinic's own cut-off is the one that applies.
The UK and US units differ too: UK laboratories report in millimoles per liter (mmol/L), and US laboratories usually in milligrams per deciliter (mg/dL). After a diagnosis, NICE recommends a review in a joint diabetes and antenatal clinic within 1 week (NICE, 2015). Management is then discussed with you by your care team.
Signs of gestational diabetes
The NHS says gestational diabetes does not usually cause symptoms and that most cases are found through screening (NHS, 2022). The CDC says that if there are symptoms they may be mild, such as being thirstier than normal or urinating more often (CDC, 2024). The NHS lists increased thirst, needing to pee more often, a dry mouth, tiredness, blurred eyesight and genital itching or thrush as possible symptoms of high blood sugar, notes that several are common in pregnancy anyway, and advises speaking to your midwife or doctor if you are worried (NHS, 2022). Because there are often no signs, the test matters more than any list.
After the birth
Does gestational diabetes go away? The NHS says it usually disappears after giving birth, but that having had it raises the chance of gestational diabetes in a later pregnancy and of type 2 diabetes in the future (NHS, 2022). The follow-up differs by country:
- UK: a blood test for diabetes 6 to 13 weeks after giving birth, and once a year after that if the result is normal. The NHS says to see a GP straight away, without waiting for the next test, if symptoms of high blood sugar develop (NHS, 2022).
- US: ACOG says to have a blood test 4 to 12 weeks after giving birth and, if it is normal, to be tested for diabetes every 1 to 3 years (ACOG, 2020).
This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.
Sources
- US Preventive Services Task Force. Screening for Gestational Diabetes: US Preventive Services Task Force Recommendation Statement. JAMA 326(6): 531-538, 2021.
- National Institute for Health and Care Excellence. Diabetes in pregnancy: management from preconception to the postnatal period (NG3). NICE, 2015.
- NHS. Gestational diabetes. nhs.uk, 2022.
- Centers for Disease Control and Prevention. Gestational Diabetes. CDC, 2024.
- National Institute of Diabetes and Digestive and Kidney Diseases. Tests & Diagnosis for Gestational Diabetes. NIDDK, 2017.
- National Library of Medicine, MedlinePlus. Glucose screening tests during pregnancy. MedlinePlus Medical Encyclopedia, 2026.
- American College of Obstetricians and Gynecologists. Gestational Diabetes (FAQ177). ACOG, 2020.
Frequently asked questions
When do you do the glucose test in pregnancy?
Usually between 24 and 28 weeks of pregnancy, in both the US and the UK (CDC; NHS). It is done earlier if you had gestational diabetes in a previous pregnancy or, in the US, if you have other risk factors. Your clinic sets the date.
Can you eat before a glucose test in pregnancy?
It depends on the test. NIDDK says you do not need to fast for the 1-hour glucose challenge test. For a glucose tolerance test you fast first: 8 to 14 hours in the US tests MedlinePlus describes, and 8 to 10 hours for the NHS test, usually with water allowed. MedlinePlus advises eating your normal food in the days before. Your clinic's instructions are the ones to follow.
What happens if you fail the 1-hour glucose test?
A high result on the 1-hour screening test is not a diagnosis. In the US two-step approach it leads to a longer glucose tolerance test, and MedlinePlus says about 2 out of 3 women who take that test do not have gestational diabetes. The clinic that ordered the test explains what a result means.
Does gestational diabetes go away?
The NHS says it usually disappears after giving birth, but that it raises the chance of type 2 diabetes later. It advises a blood test 6 to 13 weeks after the birth and once a year after that if the result is normal. ACOG gives 4 to 12 weeks after the birth, then every 1 to 3 years.