Sometime between 24 and 28 weeks of pregnancy, most women will be screened for gestational diabetes. Indeed, the recommendation in the U.S. is for universal screening. The screening involves drinking an extremely sweet drink followed by a blood test.
Given that the test is often nauseating, it’s natural to ask why it is important. Must we screen everyone? The short answer is that there are good reasons to do this, and good reasons to have the test.
Let’s get into the background on gestational diabetes, the testing, and what to do if you learn you are affected.
What is gestational diabetes?
“Gestational diabetes” is defined as diabetes that is first recognized during pregnancy. In 2020 an estimated 7.8% of births were complicated by this condition in the U.S. Women who are diagnosed with gestational diabetes actually fall into two groups.
Some had diabetes before pregnancy but it was only recognized during pregnancy. Others developed diabetes during pregnancy. The outlook for these two groups is somewhat different. For women with unrecognized diabetes before pregnancy, there are additional risks to the baby and a high likelihood that their diabetes will persist post-pregnancy. For those first diagnosed during pregnancy, the risks are lower and their diabetes is likely to dissipate after pregnancy.
For this reason, doctors are increasingly encouraging women who are at higher risk for diabetes to be tested for the first time early in pregnancy or (ideally) before. If you had a test like this, then the later test would be focused on gestational diabetes developed during pregnancy.
Anyone with high blood sugar in pregnancy, for any reason, has an increased risk of fetal overgrowth (sometimes called “large for gestational age,” or macrosomia), which can increase the risk of cesarean delivery and the risk of shoulder dystocia (when the baby’s shoulder gets caught above the mother’s pubic bone in childbirth), neonatal birth injury, and neonatal hypoglycemia (low blood sugar) after birth.
The very good news is that if high blood sugar is diagnosed during pregnancy, then changes in diet or use of insulin can dramatically lower these risks. This is why everyone is screened during pregnancy.
Anyone can develop gestational diabetes, but some groups are at higher risk, including if you:
- have a family history of Type 2 diabetes or gestational diabetes
- are older
- have a higher BMI
- have polycystic ovary syndrome (PCOS)
- had gestational diabetes in an earlier pregnancy
The same mid-trimester screening is done regardless of these risk factors.
What is the glucose test?
Generally, screening for gestational diabetes involves having a patient consume a drink that is high in sugar and then testing their blood glucose levels. Higher levels are more concerning.
There are two ways to do this test, and your doctor may use either.
The “one-step” approach: In this test, you would be asked to fast for 8 to 10 hours and then consume a drink with 75 grams of glucose. Your blood glucose level would be tested after one or two hours, and gestational diabetes would be diagnosed if the glucose level was above 180 after an hour or above 153 after two hours. This one-step approach leads directly to a diagnosis if you “fail” it.
The “two-step” approach: The first step of this test is a screening test. There is no fasting. Instead, patients consume a drink with 50 grams of glucose and their blood sugar is tested an hour later. If the glucose level is less than a cutoff (usually 135, sometimes 140), then nothing more is done. If the glucose level is above that cutoff, a second test is performed. In the second test, patients fast and then consume a 100-gram glucose solution, with a blood test three hours later. Gestational diabetes is diagnosed if the blood sugar level on the second test is too high.
There are two advantages to the two-step approach. First, most people (70% to 80%) need to do only the first step. This is a much less onerous procedure, since it doesn’t involve fasting and the drink is smaller. Second, the combination of these two procedures leads to about half as many cases diagnosed, with no difference in overall outcomes (that result is from randomized trials).
It seems that the one-step approach leads to overdiagnosis, and therefore more treatment and behavior modification than necessary.
Note: You will sometimes hear people discuss a “glucose challenge” versus “glucose tolerance.” The diagnostic tests — the one-step test, or the second step of the two-step test — are glucose tolerance tests. The screening test that is the first test in the two-step approach is often called a glucose challenge, to distinguish it.
How to prepare for the test
Regardless of which test you are having, you will generally be asked to drink a glucose drink at a specified time before you arrive at your appointment, so you are ready for the blood test at the right time. If you are having the one-step procedure or the second step of the two-step one, you will also need to fast. If you are asked to fast, it is worth asking your doctor to clarify why. Sometimes doctors will recommend that everyone fast, but there is no reason to do so for the first step of the two-step approach.
The only other thing to prepare for is that the glucose drink is pretty unpleasant for most people, but it tastes better if it is very cold.
What if I get a positive result?
If you are undergoing the two-step test and you get a positive result on the first step, remember that it is only a screening test. More than half of the people who screen positive on this test will not be diagnosed with gestational diabetes after the second step. You should ask your doctor what your glucose number is. If it is relatively low (say, 136 when the cutoff is 135), it is even more likely that you will pass the later test.
If you are ultimately diagnosed with gestational diabetes, the good news is that with the diagnosis, there are many things you can do to lower the risks in your pregnancy. Indeed, with good management, gestational diabetes carries only a small risk to the fetus. Management will involve dietary changes and monitoring of your blood sugar levels. If dietary changes are not sufficient to keep them low, you may be prescribed insulin.
For most people, gestational diabetes will resolve after the gestation ends. (You will be encouraged to screen more frequently for diabetes later in life, however, since having had gestational diabetes seems to be a marker for a higher risk of Type 2 diabetes later.)
I cannot stomach this drink. Anything else I can do?
Did we mention you could have it over ice? Unfortunately, the drink is really your only option.
Some people simply cannot keep this drink down. If you vomit, it doesn’t work. Sometimes you can use an anti-nausea drug to address this problem.
If the glucose test is impossible, you could monitor blood glucose for some period, either using finger-prick tests or a continuous glucose monitor. These are not commonly used, because they are expensive and onerous, but if the test is not possible and especially if someone seems at high risk, they may be an option.
The bottom line
- Gestational diabetes is diabetes that is first recognized during pregnancy. For some people, they had diabetes before pregnancy but it was only recognized during pregnancy. For others, they developed diabetes during pregnancy.
- Anyone with high blood sugar in pregnancy, for any reason, has an increased risk of fetal overgrowth (sometimes called “large for gestational age,” or macrosomia), which can increase birth complications. If high blood sugar is diagnosed during pregnancy, then changes in diet or use of insulin can dramatically lower these risks.
- Generally, screening for gestational diabetes involves having a patient consume a sugary glucose drink and then testing their blood glucose levels. Higher levels are more concerning.
- For most people, gestational diabetes will resolve after the gestation ends.
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