When Is the Gestational Diabetes Test Done?

If you've Googled gestational diabetes test when, you're probably either booked for it next week or wondering why a friend got tested earlier than you did. Both situations are normal, and the answer comes down to a single window plus a short list of risk factors that move the test forward.

Patients who come to me with this question usually want two things: a date on the calendar, and a sense of what the day itself looks like. This post gives you both, and adds the UK and US protocol differences so you know what your own team is following.

When Is the Gestational Diabetes Test Done? (Standard Timing)

For an average-risk pregnancy, the gestational diabetes test is done between 24 and 28 weeks. That timing is shared across NICE in the UK, ACOG and the USPSTF in the US, and most international bodies. ^[src]

Why this window? Placental hormones that resist insulin peak around the late second trimester, so testing here catches the women whose pancreas can't keep up. Test much earlier and you miss cases; test much later and you lose time to act before delivery.

The long-tail version of the question, "what week is the gestational diabetes test done," has the same answer for most readers: somewhere in those four weeks, usually pinned to your routine antenatal appointment.

OGTT vs Glucose Challenge, What Your Doctor Actually Orders

The test goes by a few names. OGTT pregnancy screening (oral glucose tolerance test) is the diagnostic gold standard, while a "glucose challenge" can mean either the full OGTT or a shorter screening step depending on where you live.

UK (NICE NG3): A single 75g, 2-hour OGTT. You fast overnight, get a fasting blood draw, drink the glucose load, then have a 2-hour draw. One step, one visit. ^[src]

US two-step (most common): A non-fasting 50g, 1-hour glucose challenge first. If your number is high, you come back for a 100g, 3-hour OGTT. ^[src]

US one-step (ADA-preferred): The same 75g, 2-hour OGTT used in the UK.

What the Test Day Looks Like (UK + US)

A 75g OGTT typically goes like this:

• The night before: Fast 8 to 10 hours. Water is fine.
• Arrival: Fasting blood draw first.
• The drink: A glucose solution finished within about 5 minutes. Most people describe it as very sweet flat lemonade.
• The wait: Sit still in the clinic. No walking, no eating, no caffeine. Bring a book or your phone.
• The draws: A 1-hour and a 2-hour draw (UK uses fasting + 2-hour; US one-step also adds the 1-hour).
• Total time: Roughly 2 to 3 hours from check-in.

Bring a snack for after the final draw. You'll be hungry, and your blood sugar will dip as the load clears.

The NHS keeps a clear plain-English version of the day on its NHS guidance on the glucose tolerance test, which is worth reading the week before your appointment.

Early GD Screening for High-Risk Pregnancies

For some women, glucose tolerance test weeks shift earlier, often to booking (around 8 to 12 weeks) or 16 to 20 weeks. This is early GD screening, high risk pathway, and it matters for a meaningful slice of patients.

The American Diabetes Association's 2026 Standards of Care recommend screening before 15 weeks if any of the following apply: ^[src]

• Previous gestational diabetes
• BMI ≥ 30
• Previous baby weighing over 4kg / 9lb at birth
• First-degree relative with type 2 diabetes
• Polycystic ovary syndrome (PCOS)
• South Asian, Black, Hispanic, Native American, or Pacific Islander ancestry
• Known prediabetes or HbA1c ≥ 5.7%

NICE NG3 in the UK is slightly different: an early OGTT at booking if you have risk factors, repeated at 24 to 28 weeks if the first is normal.

A common question I get: "Do I need an early GD test with PCOS?" The honest answer is yes, ask for it. PCOS roughly doubles the GD risk, and we'd rather start watching at 12 weeks than discover a problem at 30.

What Your Numbers Mean (Result Cutoffs)

The cutoff numbers vary by protocol. Here's what triggers a diagnosis:

NICE NG3, 75g OGTT: fasting ≥ 5.6 mmol/L OR 2-hour ≥ 7.8 mmol/L. One abnormal value diagnoses GD.

ADA / IADPSG one-step 75g: fasting ≥ 92 mg/dL, 1-hour ≥ 180 mg/dL, 2-hour ≥ 153 mg/dL. One abnormal value diagnoses GD.

US two-step (Carpenter-Coustan, 100g, 3-hour): fasting ≥ 95 mg/dL, 1-hour ≥ 180 mg/dL, 2-hour ≥ 155 mg/dL, 3-hour ≥ 140 mg/dL. Two or more abnormal values diagnose GD.

If your number sits just above the cutoff, ask whether your team treats it as definite GD or as borderline needing a recheck. Both approaches exist.

What Happens If You're Diagnosed

A diagnosis doesn't mean immediate medication. The first step is almost always:

1. A 1 to 2 week trial of diet and physical activity changes. The full plan is in our gestational diabetes diet plan.
2. A glucose meter, with finger-prick readings four times daily: fasting plus 1 hour after each main meal.
3. A referral. In the UK, that's a diabetes specialist midwife and dietitian; in the US, it's often a maternal-fetal medicine specialist or diabetes educator.

If your numbers stay above target after the trial, metformin or insulin can be added safely in pregnancy. Many women never need medication.

What This Means for You

The gestational diabetes test when most pregnancies happens between 24 and 28 weeks, with earlier screening if you have PCOS, prior GD, high BMI, or other risk factors. Knowing your protocol (UK 75g one-step, US one-step, or US two-step) helps you arrive prepared.

If you've been diagnosed or you're waiting for results, a quick consultation can give you a clearer plan for the next two weeks. You can also read what to expect for long-term effects of GD on baby, and where many women land emotionally with did I cause my gestational diabetes.

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