Glucose Tolerance Test During Pregnancy: What to Expect and How to Prepare

The glucose tolerance test is one of the most commonly performed tests in pregnancy, and one of the most frequently misunderstood. Women arrive at the appointment uncertain about whether they should have eaten breakfast, anxious about what a failed result means, and unclear about what the test is actually measuring.

This article answers all of that — practically and directly. Understanding what the test is for, exactly what happens during the appointment, and what the results mean makes the experience significantly less stressful and the information it produces significantly more useful.

What the glucose tolerance test is for

The glucose tolerance test (GTT) — also called the oral glucose tolerance test (OGTT) — screens for gestational diabetes: a form of diabetes that develops during pregnancy when the hormones of the placenta cause the body to become resistant to insulin.

The test works by giving you a measured amount of glucose — a sugary drink — and then measuring how your blood glucose levels respond over time. In a person with normal insulin function, blood glucose rises after the glucose drink and then returns to a normal range within one to two hours as insulin moves the glucose into cells. In a person with insulin resistance, blood glucose rises higher than it should and stays elevated for longer.

The test is not measuring something rare. Gestational diabetes affects a significant proportion of pregnant Indian women — estimated at fourteen to twenty-one percent in urban populations — and the consequences of missing it are real. A positive result allows management that significantly reduces risks to both mother and baby.

Who gets tested and when

Timing: For most women, the glucose tolerance test is performed between twenty-four and twenty-eight weeks of pregnancy — the period when placental hormones that cause insulin resistance are at levels sufficient to trigger gestational diabetes.

High-risk women are tested earlier and more than once: Women with risk factors — previous gestational diabetes, strong family history of diabetes, polycystic ovary syndrome (PCOS), obesity, or significantly elevated blood sugar in early testing — may be offered testing in the first trimester, repeated at twenty-four to twenty-eight weeks if the first result is negative.

In India, universal testing is common: Given the high prevalence of gestational diabetes in the Indian population, many providers and facilities test all pregnant women rather than only those with identified risk factors.

The different versions of the test

There are two main approaches to gestational diabetes screening in India, and knowing which one you are having helps you prepare correctly.

The 75g OGTT (one-step approach) — recommended by the Diabetes in Pregnancy Study Group India (DIPSI) and most commonly used in India. This is a fasting test. You arrive having fasted overnight (typically eight to twelve hours), a fasting blood sample is drawn, you drink 75 grams of glucose dissolved in 250–300 ml of water, and blood samples are taken at one hour and two hours after drinking.

Positive (gestational diabetes diagnosis) if any one of the following thresholds is met:

Fasting blood glucose ≥ 92 mg/dL
One-hour blood glucose ≥ 180 mg/dL
Two-hour blood glucose ≥ 153 mg/dL

The 50g glucose challenge test (GCT) followed by 100g OGTT (two-step approach) — less commonly used in India than in the US, but you may encounter it. The GCT is non-fasting — you don’t need to fast, you drink 50 grams of glucose, and a single blood sample is taken one hour later. If the one-hour result is elevated (typically ≥ 130–140 mg/dL depending on the threshold used), you return for a full three-hour, 100g OGTT on a separate day. The full OGTT requires two or more abnormal values for a gestational diabetes diagnosis.

Ask your provider which version you are having so you can prepare correctly.

How to prepare

For the fasting 75g OGTT (the most common version in India):

Fast for eight to twelve hours before the test. Water is permitted; nothing else — no chai, no coffee, no food, no glucose-containing drinks.
Continue taking your usual medications unless specifically instructed otherwise by your provider.
Do not exercise intensely on the morning of the test or the evening before — intense exercise can temporarily affect glucose metabolism.
Eat a normal, balanced diet in the days before the test. There is no specific preparation diet needed. Do not eat less carbohydrate than usual in the preceding days in an attempt to lower the result — this actually impairs your body’s glucose-processing capacity and can produce a falsely abnormal result.
Plan to be at the testing facility for at least two to three hours. You will be asked to remain there between blood draws.
Bring something to occupy yourself — a book, your phone. Some women find the wait uncomfortable; having something to focus on helps.
Bring a small, balanced snack for after the test, for when the fast is broken.

For the non-fasting 50g GCT:

No fasting required. You can eat normally before this test.
The appointment is shorter — one blood draw approximately one hour after the glucose drink.

What happens during the appointment

The experience of the 75g OGTT, which involves waiting between blood draws, is what most women want to know about in advance.

Arrival: You arrive fasted. The registration and initial assessment typically involve a brief review of your pregnancy details and confirmation that you have fasted appropriately.

Fasting blood draw: A blood sample is taken before the glucose drink — this is the fasting glucose measurement.

The glucose drink: You will be given a measured glucose solution — 75 grams of glucose in approximately 250–300 ml of water. It is sweet, can feel intense in sweetness first thing in the morning on an empty stomach, and must be consumed within five minutes.

The wait: You remain at the facility for two hours following the glucose drink. You will be asked to sit quietly and rest — significant physical activity between the draws can affect results. You cannot eat or drink anything other than water during this period.

Blood draws at one hour and two hours: Blood is taken at each interval. These are the measurements that, alongside the fasting value, determine the result.

Common experiences during the test: Some women feel nauseated after the glucose drink, particularly if they are in the first trimester or have had morning sickness. Some feel light-headed or faint, particularly if the fast has been long. Some feel fine throughout. Tell the testing staff if you feel unwell. If you vomit after the glucose drink before the test is complete, let your provider know — the test may need to be rescheduled.

Understanding the results

Results are typically available the same day or the following day and are sent to your provider.

A normal result means all three blood glucose values are below the diagnostic thresholds. This means gestational diabetes has not been identified at this test. If you are tested at twenty-four to twenty-eight weeks with a normal result, testing is generally not repeated unless new risk factors develop.

An abnormal result — meeting one or more of the diagnostic thresholds — means gestational diabetes is diagnosed. This leads to:

A referral or consultation with your obstetrician and possibly a diabetologist or dietitian
Home blood glucose monitoring, usually four times daily (fasting and after each main meal)
Dietary modifications — the companion article on gestational diabetes and Indian food covers this in detail
Physical activity guidance
Medication (oral or insulin) if dietary management is insufficient to control blood glucose

A borderline result — just below the diagnostic threshold but higher than typical — may lead to closer monitoring, repeat testing, or dietary advice, depending on your provider’s clinical judgment.

What a positive result actually means for your pregnancy

A gestational diabetes diagnosis changes how your pregnancy is monitored and managed, but it does not mean your pregnancy has become a crisis. Most women with gestational diabetes manage their blood glucose effectively through dietary changes and, where needed, medication, and go on to have healthy babies.

It does mean:

More frequent antenatal appointments
Additional ultrasound scans to monitor fetal growth (because elevated glucose can cause the baby to grow larger than expected)
A more planned approach to the timing and method of delivery if the baby is very large or blood glucose control has been difficult
A glucose test six to twelve weeks after delivery to confirm resolution, and annual monitoring thereafter given the long-term risk of Type 2 diabetes

What it does not mean:

That you caused this through diet or lifestyle choices before pregnancy
That you will need insulin (many women manage with diet alone)
That your baby will have diabetes
That a caesarean section is automatically necessary

The honest message about the test itself

The glucose tolerance test is uncomfortable for some women — the sweet drink, the fasting, the long wait. It is not dangerous, and the discomfort is temporary.

What it provides is information that meaningfully affects the care of a pregnancy — either the reassurance of a normal result or the early identification of gestational diabetes at a point when management is most effective. Given that India has among the highest rates of gestational diabetes in the world, and that the condition often has no symptoms that would otherwise prompt investigation, the GTT is doing genuinely important work.

Go fasted. Drink the drink. Wait. Get the result. And if the result is not what you hoped, know that the next steps are manageable — and that knowing early is always better than finding out late.

This article is for general educational purposes only and does not replace personalised medical advice. Always consult your doctor, midwife, or a qualified healthcare professional about glucose testing and gestational diabetes management in your pregnancy.