Glucose Tolerance Test in Pregnancy: How Should You Eat Beforehand?

You do not need to follow a special diet before a glucose tolerance test in pregnancy; on the contrary, it is recommended that in the days before the test you carry on with your usual balanced eating without cutting carbohydrate. On the morning of the test you fast for the time your laboratory and doctor tell you (often 8–10 hours); during this time you are generally allowed to drink only water.

Some expectant mothers give up bread, rice and fruit in the days before the test so that “the result will be good”. This approach can distort the result. Below you will find, step by step, what to watch from the days before the test to after it.

What Is the Glucose Tolerance Test and When Is It Done?

The glucose tolerance test is a blood test that measures how your body handles a set amount of glucose. Its aim is to detect early the condition that can arise in pregnancy and is called gestational diabetes.

In the oral glucose tolerance test (OGTT) described by the NHS, blood is taken in the morning on an empty stomach, then you drink a sugary drink and, after resting for two hours, blood is taken again. The test is usually done between weeks 24 and 28. The American College of Obstetricians and Gynecologists (ACOG) also states that all pregnant women should be screened for gestational diabetes and that in those at risk the test can be done earlier in pregnancy.

In practice, one-step (a single glucose load) or two-step (first a short screening test and, if needed, a longer load) tests can be used. Which test is done, in which week and how is decided by your obstetrician; the suggestions in this article do not replace the instructions of your doctor and the laboratory.

How Should You Eat in the Days Before the Test?

In the days before the test, eat regularly and in a balanced way as usual; do not cut carbohydrate, and do not overdo it either. Keep eating sources of carbohydrate such as bread, bulgur, rice, potatoes, fruit and dairy products in your usual portions.

There is a scientific reason for this. In a study of pregnant women, blood sugar values after the glucose load rose as the carbohydrate eaten in the 24 hours before the test fell; the researchers concluded that restricting or greatly increasing carbohydrate before the test can affect the diagnosis of gestational diabetes. In other words, the thought of “let me cut sugar before the test” can, contrary to expectation, lead to a high result and unnecessary worry.

• Do not skip meals; carry on with three main meals and snacks as needed.
• Have a portion of wholegrains or another carbohydrate source at every meal.
• Do not change your usual pattern with sweets, sugary drinks and fruit juice either; eating a lot of sweet food “one last time” before the test is not right.
• If your doctor has recommended a special preparation, follow it.

A sample menu for the day before the test

• Breakfast: Cheese, a boiled egg, tomato, cucumber, olives and two slices of wholemeal bread
• Snack: A medium piece of fruit and a bowl of yoghurt
• Lunch: A vegetable dish with meat or cooked in olive oil, four or five tablespoons of bulgur pilaf, salad
• Snack: A slice of bread with cheese, or a handful of nuts and dried fruit
• Dinner: Lentil soup, grilled chicken or fish, boiled potatoes or pasta, salad

This menu gives an example of an ordinary day in pregnancy that neither cuts carbohydrate nor overdoes it. If you can only eat a little because of nausea, or if you have a special eating plan your doctor recommended, carry on with it unchanged before the test and let your doctor know.

What Do You Eat and Drink the Night Before and the Morning of the Test?

Eat dinner in a balanced way as usual and finish it before the fasting period begins. The NHS advises not eating or drinking anything for 8–10 hours before the OGTT, although water can often be drunk; if you are not sure, ask the hospital.

Ask your doctor in advance whether you should take the medicines you use regularly on the morning of the test. Do not skip medicines on your own.

What Should You Watch During the Test?

During the test you should not eat, should rest sitting down as far as possible and should not smoke. Moving about and eating can affect blood sugar values.

• Drink the sugary drink within the time you are told; drinking it too fast can increase nausea.
• If you vomit after the drink, tell the staff straight away; the test may be considered invalid and may need to be repeated.
• Having a book or headphones with you makes the waiting time easier.
• If you feel dizzy or unwell, say so straight away.
• Bring a small, balanced snack with you to eat when the test is over.

What Should You Eat After the Test?

Eating a balanced meal after a long fast and the sugary drink helps you feel better. A meal that includes protein, fibre and wholegrains helps blood sugar run more evenly.

For example, a breakfast with egg, cheese, tomato and cucumber and wholemeal bread, or yoghurt, walnuts and a portion of fruit, are good options. Rather than filling up as soon as you leave the test on quickly digested options such as poğaça (Turkish savoury bun), simit (Turkish sesame bread ring) and fruit juice, choose balanced options like these. You can find how to build a healthy plate in pregnancy on my page on nutrition during pregnancy.

Who Can Have the Test Earlier?

In expectant mothers at higher risk of gestational diabetes, the test can be done in earlier weeks of pregnancy and repeated at weeks 24–28. The risk factors listed by the NHS are:

• A body mass index above 30
• Having previously had a baby weighing 4.5 kg or more
• Having had gestational diabetes in a previous pregnancy
• Diabetes in a first-degree relative
• Being of South Asian, Black, African-Caribbean or Middle Eastern origin
• Having previously had weight-loss (stomach reduction) surgery

Remember that pre-pregnancy weight and weight gain in pregnancy are also linked to the risk of gestational diabetes; you can look at the article on how much weight to gain in pregnancy for the expected ranges.

What Happens If the Result Is High?

If you are diagnosed with gestational diabetes, your doctor will give you advice on blood sugar monitoring, changes to eating and activity, and plan medication if needed. When gestational diabetes is detected early and well managed, the risks for mother and baby can be reduced.

Eating with gestational diabetes involves spacing meals at regular intervals, choosing the right sources and portions of carbohydrate, and adding protein and fibre to every meal. ACOG states that a routine of three main meals and two or three snacks is generally recommended, and that a 10–15 minute walk after meals may help blood sugar control. You can read the gestational diabetes nutrition guide for detailed advice.

For preparation before the test or an individual nutrition plan after a diagnosis of gestational diabetes, we can work together, in line with your obstetrician's advice, within nutrition counselling for pregnancy.

Sources

• NHS – Gestational diabetes
• ACOG – Gestational Diabetes (FAQ)
• Relationship between carbohydrate intake and oral glucose tolerance test results among pregnant women. Diabetes Res Clin Pract, 2021 (PubMed)

This article is for general information only; please consult your doctor and dietitian about your own situation.