Glucose Tolerance Test in Pregnancy
The glucose tolerance test, also commonly referred to as the blood sugar curve in everyday language, is a routine test performed in almost all pregnancies during the second trimester.
Its purpose is to determine whether the body responds properly to insulin during pregnancy and to detect gestational diabetes early, if present.
It is not a test you should be afraid of, but it is worth understanding well so you can go prepared.
What is the glucose tolerance test and why is it done during pregnancy?
During pregnancy, the hormones produced by the placenta can reduce the body’s sensitivity to insulin, meaning they can cause insulin resistance.
In most women, the pancreas compensates for this change by producing more insulin. When this does not happen adequately, blood sugar levels rise and gestational diabetes develops.
Gestational diabetes often causes no symptoms, which is why the test is necessary. If it is not diagnosed and treated early, it can affect both the mother’s health and the baby’s development.
When is the glucose tolerance test done during pregnancy?
The glucose tolerance test is usually performed between the 24th and 28th week of pregnancy — the period when pregnancy-related insulin resistance is at its highest.
In some cases, the test is done earlier, even in the first trimester, when there are high-risk factors for gestational diabetes, such as:
- High body mass index (BMI > 30)
- Family history of type 2 diabetes
- Gestational diabetes in a previous pregnancy
- Polycystic ovary syndrome (PCOS)
- Age over 35
How is the glucose tolerance test done — the process step by step
The test is done in the morning, while fasting, and lasts a total of 2 hours. What exactly happens:
First measurement — Fasting
As soon as you arrive, a blood sample is taken to measure your fasting blood sugar (the baseline reference value).
Drinking the glucose solution
You then drink a glucose solution, usually 75 g dissolved in water. The taste is sweet, similar to lemonade, and it must be consumed within 5 minutes.
Waiting period
You stay at the lab resting for 2 hours (you do not eat, you do not drink anything else, and you avoid strenuous movement).
Second and third measurement
New blood samples are taken after 1 hour and after 2 hours to see how your body processes glucose over time.
Preparing for the glucose tolerance test — what to do beforehand
Proper preparation is critical for reliable results.
Fasting
A fast of 8–12 hours is required before the test — you may drink only water.
What to eat the night before the glucose tolerance test
You do not need to drastically change your eating habits.
Eat dinner as usual, but avoid meals high in simple sugars and soft drinks. A balanced dinner with protein, vegetables, and complex carbohydrates is the best choice. Avoid sweets, fruit salads, and juices.
Medication
If you are taking any medication, inform your doctor in advance. Some medicines may affect the results.
Possible side effects after the glucose tolerance test
After drinking the glucose solution, some women may feel:
- Dizziness / vertigo or mild discomfort — this is the most common sensation, especially if the stomach is completely empty
- Nausea, which usually passes quickly
- Feeling weak or tired — expected after fasting and drinking glucose
- Headache — less common, but possible
These sensations are temporary and usually subside once the test is completed and you eat something. If the dizziness or discomfort is intense during the waiting period, inform the staff immediately.
What the results show | Normal values
The results are evaluated based on the following normal ranges:
- Fasting: < 92 mg/dl
- 1 hour after glucose: < 180 mg/dl
- 2 hours after glucose: < 153 mg/dl
If one or more values exceed these limits, gestational diabetes is diagnosed. This does not mean panic — it means that management is needed, which in most cases involves dietary changes and monitoring.
Frequently Asked Questions (FAQ)
Yes, water is allowed during fasting and does not affect the results. Avoid anything else, including coffee, even without sugar.
In most cases, yes, but it is recommended to eat something immediately after the test before driving. If you feel dizzy or weak, it is best to wait until it passes before getting into the car.
The next steps are discussed individually and usually include dietary guidance, self-monitoring of blood sugar at home, and closer follow-up. In most cases, gestational diabetes is managed effectively with dietary changes, without the need for medication.
No, the test is completely safe for the baby. The glucose solution you drink does not pose a risk to the pregnancy.
If the nausea is severe, inform your doctor before the test. In some cases, an alternative screening method may be preferred or the test may be postponed for a few days.
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