The glucose tolerance test (OGTT): what to expect and how to prepare

During pregnancy the body handles sugar a little differently to meet your baby's needs. For most people this balance works smoothly, while for some blood sugar can rise because of the pregnancy itself. The glucose tolerance test is a common way to spot this early.

What is gestational diabetes?

Gestational diabetes is a rise in blood sugar that appears for the first time during pregnancy and usually settles after birth. Hormones made by the placenta can weaken the effect of insulin. If the body cannot keep up, blood sugar goes up.

It often causes no symptoms. That is why screening is offered even if you feel well. Once it is found, it can usually be managed well with eating habits and follow-up.

When is the test done?

If you are not in a higher-risk group, the test is usually done between weeks 24 and 28. If you have risk factors, your doctor may suggest it earlier or with a different plan. Timing can vary between clinics, so your doctor decides the exact week.

How is the test done?

The details differ a little between sources and clinics. Two common approaches exist:

• One-step (75 g) test: A fasting blood sample is taken, you drink a sugary solution, and blood is drawn again at set times (often at 1 and 2 hours).
• Two-step approach: First a short screen (often 50 g) that does not require fasting. If the result is above the cut-off, a longer glucose test (often 100 g) follows.

Your doctor or lab will tell you which one applies. The drink is very sweet and can cause nausea in some people.

How should you prepare?

The instructions from your lab come first. In general:

• Eat as you normally do in the days before; cutting back on sugar and carbohydrates can distort the result.
• For longer tests, fasting for about 8–12 hours is usually asked; only water is allowed.
• You will be there for 2–3 hours, so bring a book, water and a snack for afterwards.
• Pick a morning when you are not in a rush; you may be asked to stay seated and avoid walking around.

What do the results mean?

Cut-off values depend on the method used, so rather than interpreting one number alone, look at the reference ranges on the lab report and at your doctor's assessment.

A screening result above the cut-off is not a diagnosis. A second test is usually requested. If you are diagnosed, it is not the result of anything you did or chose; it is linked to the hormonal changes of pregnancy.

What you can do

• Call the lab beforehand and ask about preparation and duration.
• Bring someone along if you can; tiredness or dizziness afterwards is easier with company.
• If a result is borderline, try not to panic; many people have a normal result on retesting.
• If you are diagnosed, ask your doctor or a dietitian for support with eating and blood sugar monitoring.
• You can browse the Can I eat it? section of the app, but a personal eating plan is best made with your care team.

When to contact your doctor

• If you have severe nausea or vomiting and cannot keep the drink down
• If you notice extreme thirst, very frequent urination or marked weakness
• If you feel dizzy, faint, shaky or sweaty
• If your baby's movements clearly decrease
• If anything about the timing or results of the test worries you

In a severe situation call 112 (or your local emergency number). This article is for information; your doctor's advice for your own situation comes first.