Glucose Tolerance During Pregnancy

During pregnancy, there is a high risk of elevated blood sugar levels. To prevent this, a special glucose tolerance test is conducted. It helps determine how ‘tolerant’ the expectant mother’s body is to glucose and whether there is any negative impact. If the level of tolerance in the blood is above normal, special treatment is prescribed. 

In the sixth month of pregnancy, glucose tolerance is determined using a special test. This helps identify hidden diabetes, which can be found in 3-6% of pregnant women. It is commonly believed that gestational diabetes only occurs in pregnant women because it is provoked by pregnancy hormones. In other words, it disappears immediately after delivery, and the future mother won’t have to suffer from it throughout her life. 

Checking glucose tolerance during pregnancy is very important, as it helps prevent a number of undesirable outcomes, including the birth of a large fetus or prenatal complications. Another name for the glucose tolerance test is the ‘O’Sullivan Test’ after the scientist who invented it. There is no alternative to the GTT with similar efficiency, although occasionally different methodologies are used. 

What is the glucose tolerance test

Gestational diabetes mellitus is a condition where glucose tolerance is impaired in the body. It occurs or is diagnosed during pregnancy. This disorder does not fall within the scope of regular diabetes mellitus, but it is still a serious medical issue. When glucose tolerance is impaired, specific health risks are established for both the mother and the fetus. Moreover, there is a high likelihood that the woman will develop type 2 diabetes in the future.  

During pregnancy, insulin resistance increases significantly. Additionally, compensatory insulin secretion increases and hyperinsulinemia may even occur. During the first trimester and the first half of the second trimester, blood glucose levels are lower than in non-pregnant women. It is only by the second trimester that insulin resistance occurs. Throughout the entire pregnancy, this resistance gradually increases. 

This phenomenon has a specific physiological meaning. It’s about adequately supplying glucose to the fetus. This mechanism is largely dependent on the influence of hormones secreted by the placenta. If gestational diabetes occurs, changes in insulin resistance become more pronounced compared to similar indicators during a normal pregnancy.  

The test allows determining the level of sugar in the blood and how receptive the mother’s and fetus’s bodies are to glucose, the consequences of its impact. 

When to Conduct GTT

There are several factors that determine when to conduct a glucose tolerance test:

• if doubtful glycemic values are detected (allows clarification of carbohydrate metabolism state);
• if there are risks of developing diabetes;
• BMI over 25 kg/m2;
• diabetes is present in family history;
• low physical activity on a regular basis;
• blood pressure issues;
• disrupted lipid metabolism;

In turn, it is also important to highlight several factors when the test is prohibited:

• presence of any acute, including infectious, disease;
• in case of necessity to take medications that lead to increased glycemia levels (ideally, discontinue them three days before the GTT);
• gestational age over 32 weeks;

It should be remembered that this test is not always required. Typically, it is prescribed on an individual basis, after the attending physician evaluates the overall condition and tests of the expectant mother. 

How to prepare for the test

The glucose tolerance test is conducted while maintaining a usual diet. There is only one condition: carbohydrate intake must not be reduced to less than 150 grams per day for at least 3 days before the test. The GTT is conducted in the morning on an empty stomach, with the last meal taken 8-14 hours before the test. The dinner on the eve of the test should contain 30-50 grams of carbohydrates. 

During the test, drinking water is allowed, and the pregnant woman should be in a sitting position. Smoking is strictly prohibited until the test is completed. Additionally, before the glucose tolerance test, any medications that could affect the results are prohibited. 

Test Stages

The glucose tolerance is determined in two stages:

1. Phase #1. Up to 24 weeks (usually right after pregnancy confirmation), one of the following tests is mandatory: fasting venous plasma/serum glucose intake, glycated hemoglobin test, venous plasma/serum glucose at any time of day regardless of food intake.
2. Phase #2. A GTT (Glucose Tolerance Test) is conducted at 24-28 weeks of pregnancy. This phase is mandatory for all pregnant women exposed to risk factors. This includes patients with undetected disorders in Phase 1.

The glucose tolerance test during pregnancy has specific characteristics that must also be considered.  

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Standards and evaluation of test results

Only a doctor can interpret the results of the GTT – the obstetrician, therapist, endocrinologist. The norm for glucose tolerance during pregnancy is determined by the following indicators: fasting – less than 5.1 mmol/l, after an hour – less than 10 mmol/l, after two hours – 8.5 mmol/l. If, after testing, at least one of these values is exceeded, it already indicates a diagnosis of GDM.

Attention should also be paid to the presence of reference values. Thus, reference values ​​for blood plasma glucose when measured on an empty stomach during pregnancy range from 4.1 to 5.1 mmol/l. Main criterion of norm:

There are several reasons why glucose tolerance may be increased:

• disturbed carbohydrate metabolism;
• false positive result due to acute stage illness, surgical operations or any other stressful situation;
• taking medications that increase glycemia levels.

It is important to separately consider why glucose tolerance might be elevated during pregnancy: taking medications that lower glycemia levels (insulin, various hypoglycemic drugs), insulinoma, excessive fasting, or intense physical exertion before the test. 

Alternatives to GTT

GGT is not performed if there are any acute conditions. Even a common cold can fall under this category. Such changes strongly affect the test results. In addition, it’s worth highlighting several reasons that might cause false results when determining glucose tolerance. 

This is possible if the eight-hour fasting is not followed before the blood test. Additionally, if there were significant changes in diet, such as increased or decreased carbohydrate intake three days before the test, this can also affect the GTT results. Carbohydrate metabolism disorders, excessive physical activity, intense stress situations, and infectious diseases — all these directly influence the final results. 

Depending on the method of glucose load introduction into the body, there are two types of GTT — oral and intravenous. The latter is used if, during pregnancy, the expectant mother cannot, for some reason, consume the “sweet cocktail” orally. The test itself poses no danger. 

In cases where the test is conducted with a load, there is a risk of overdosing the blood sugar level. This is because the pregnant woman already has a high level of glucose — symptoms indicating impaired carbohydrate metabolism will begin to appear.

The glucose tolerance test is done for a reason. The test is only performed if there are serious suspicions of diabetes. There are no equivalents to this test, but considering that blood is taken once a trimester as a mandatory procedure, one can find out the blood sugar level without additional load.

Recommendations for Pregnant Women

In cases of impaired glucose tolerance, a special diet must be followed. It is determined taking into account the pregnant woman’s body mass index and her taste preferences. This helps maintain normal sugar levels. In case a special diet is prescribed, the complete exclusion of simple carbohydrates and limited consumption of complex carbohydrates is recommended. 

Additionally, it is necessary to consume a sufficient amount of protein — meat, fish, seafood, dairy and fermented milk products. The diet during pregnancy should be enriched with fiber and vitamins — greens, vegetables, sour-sweet berries. Special attention should be paid to the correct choice of fats in accordance with the amount established by a doctor. Animal fats are completely restricted. 

Instead of frying and using a deep fryer, dishes should be boiled, stewed, baked, and steamed. Upon detecting gestational diabetes, the doctor will create a special menu that should be followed until the end of the pregnancy. The individual characteristics of each woman are necessarily taken into account. 

A pregnant woman should not use menu #9 in its pure form due to its significant calorie restrictions. The doctor will provide detailed information about the diet plan during an in-person consultation. If the desired level of glucose tolerance cannot be achieved through diet, and if there is a negative impact on the fetus, medications, specifically insulin, are prescribed. 

Pregnant women should not use oral hypoglycemic agents. Insulin therapy is prescribed by an endocrinologist. Throughout the pregnancy, women must be under the continuous supervision of three doctors: an endocrinologist, a therapist, and an obstetrician-gynecologist. Patients must adhere to regular physical activity. This can include walks in the fresh air, special yoga, or swimming.