Oral glucose tolerance test (OGTT) – what is it and how to interpret the results?

The oral glucose tolerance test (OGTT) during pregnancy is also referred to as the sugar load test. This test is mandatory during the second trimester of pregnancy but may sometimes be performed earlier to diagnose any deviations in blood sugar levels as soon as possible. Untreated gestational diabetes poses the risk of serious complications.

Gestational diabetes affects 3 to 5% of all pregnant women. To stop its development or balance blood sugar levels, the mandatory screening test, known as the oral glucose tolerance test (OGTT), must be carried out first. Pregnant women should not delay this test, as gestational diabetes can present significant risks for both the mother and the baby. It is recommended to perform the OGTT between the 24th and 28th week of pregnancy.

Oral glucose tolerance test during pregnancy – when should the test be done?

In Switzerland, the oral glucose tolerance test (OGTT) is one of the important screening examinations during pregnancy. It is essential that every expectant mother knows when the OGTT should be performed during pregnancy. If a woman had no diabetes prior to pregnancy, it is recommended to take the test between the 24th and 28th week of pregnancy. However, women at higher risk for gestational diabetes will undergo the OGTT earlier, with the timing of the test determined by the attending physician.

Risk factors for the development of gestational diabetes include:

• Age over 35 years
• Obesity
• High blood pressure
• Gestational diabetes in previous pregnancies
• PCOS (polycystic ovary syndrome)
• Positive family history
• Macrosomia, meaning a birth weight that is too high relative to the gestational age

How does the oral glucose tolerance test work?

No special preparation is required to obtain reliable results from the OGTT. There should be no changes in diet or increased physical activity. The pregnant woman must be healthy, as even a mild cold can affect the test results. The doctor should be informed about any medications being taken, especially beta-blockers or psychotropic drugs. The test should be conducted after fasting, meaning no food or drinks should be consumed for at least 8–10 hours prior to the test.

The oral glucose tolerance test (OGTT) is a multi-stage examination. First, the fasting blood glucose level is measured. Then, a glucose solution is prepared for the pregnant woman to drink. The blood glucose levels are measured one hour and two hours after drinking a solution with 75g of glucose.

Interpreting the results of the oral glucose tolerance test (OGTT)

Normal values for the OGTT during pregnancy:

1. Fasting test – under 92 mg/dl
   A value of 92-125 mg/dl indicates gestational diabetes.
2. Blood glucose measurement after one hour: under 180 mg/dl
3. Blood glucose measurement after two hours: A normal blood sugar level is under 153 mg/dl.

If the blood glucose level in the OGTT is above 126 mg/dl after fasting or above 200 mg/dl after two hours, gestational diabetes (GDM) is diagnosed.

Treatment of gestational diabetes

After diagnosis, the pregnant woman must promptly visit a diabetologist. It is necessary to change her previous habits. The daily calorie requirement for a pregnant woman depends on her BMI prior to pregnancy, but it generally ranges between 1800 and 2500 kcal. The diet should be well-balanced, consisting of three main meals and three snacks (including one before bedtime). The diabetic diet should contain approximately 40-45% carbohydrates, 30% protein, and 20-30% fat.

Moderate physical activity is also recommended, tailored to the pregnancy. Pregnant women with gestational diabetes must measure their blood glucose levels daily on an empty stomach and one hour after each meal. If dietary changes and optimal physical activity are not enough, insulin injections are required. Pregnant women will administer insulin to themselves at home.

Complications of gestational diabetes

The complications can be divided into two groups: maternal and fetal complications.

Maternal complications:

• Urinary tract infections
• Conversion of gestational diabetes into type 2 diabetes after childbirth
• Polyhydramnios (excessive amniotic fluid)
• Preeclampsia (pregnancy-induced hypertension)
• Pyelonephritis (kidney infection)
• Edema (swelling)

Fetal complications:

• Congenital malformations
• Malformations of the cardiovascular system
• Macrosomia (excessive weight gain of the baby relative to gestational age)
• Nervous system malformations
• Intellectual disabilities
• Digestive system malformations
• Limb malformations

In cases of macrosomia, a natural birth is not possible, and the delivery is scheduled around the 37th week of pregnancy for a cesarean section. This is related to the accelerated growth of the baby in the womb due to the high blood sugar levels. The newborn may be at risk of hypoglycemia (low blood sugar) immediately after birth, which in extreme cases can lead to a hypoglycemic coma. Later in life, macrosomia can lead to metabolic disorders, overweight, obesity, impaired glucose tolerance, high blood pressure, and insulin resistance