The gestational diabetes test: what to know about the OGTT

Somewhere around the middle of pregnancy, you will probably hear about the “glucose test”. For some women it is a minor inconvenience; for others, the fasting, blood tests or sweet drink are a real challenge. Here is what it checks and why it gets recommended.

What is gestational diabetes?

Gestational diabetes happens when blood glucose levels become higher than they should be during pregnancy. Pregnancy hormones can make insulin work less effectively. It is not a judgement on what you eat or how well you have cared for yourself.

Untreated high blood glucose can increase the likelihood of complications for you and your baby, including a larger baby, birth complications and high blood pressure conditions. Identifying it makes monitoring and treatment possible.

Why is the OGTT offered at 24–28 weeks?

The 2025 recommendations from the Australasian Diabetes in Pregnancy Society (ADIPS) advise offering all pregnant women without previously identified diabetes a 75 g, two-hour oral glucose tolerance test (OGTT) at 24–28 weeks. Some women are offered earlier testing because of individual risk factors.

For the usual OGTT you fast beforehand (follow your pathology provider’s instructions), have a blood sample taken, drink a 75 g glucose drink, then have more blood taken one and two hours later. Plan to be at the collection centre for a little over two hours and bring something to eat afterwards.

What if I feel sick or cannot manage the test?

Tell your midwife or GP before the appointment if you have severe nausea, difficulty fasting, a history of bariatric surgery or concerns about the test. If you vomit during the test, the laboratory may need to stop and arrange another plan.

The 2025 ADIPS guidance suggests a fasting plasma glucose test when an OGTT is not undertaken, but it specifically cautions that a normal fasting result cannot rule out gestational diabetes. Finger-prick readings or HbA1c are not simply interchangeable with the standard OGTT. Ask your care provider what alternative testing and follow-up would mean in your circumstances.

If your result is above the diagnostic threshold

A gestational diabetes diagnosis often means education about food and activity, checking blood glucose and extra review during pregnancy. Some women need medication, including insulin. Your treatment and birth discussions should be based on your actual glucose readings, baby’s growth and other factors, not the diagnosis alone.

Questions to take to your appointment

• Am I being offered routine testing, or is there a specific concern?
• How should I prepare, and what happens if I cannot keep the glucose drink down?
• If I do not have an OGTT, what can the alternative tests tell us — and what can they not tell us?
• If I am diagnosed, who will help me understand the results and my care options?

For the clinical detail behind this recommendation, see the ADIPS 2025 consensus statement.

Further reading

• Gestational diabetes — healthdirect
• ADIPS 2025 consensus recommendations — Medical Journal of Australia

This article is general education, not personal medical advice. Your midwife, GP or obstetrician can help you understand what applies to your individual circumstances.