What is the glucose tolerance test for?
The glucose tolerance test, often called the GTT, checks how well your body processes sugar during pregnancy and screens for gestational diabetes — a form of high blood sugar that develops during pregnancy and usually resolves after birth. It works by having you drink a sugary liquid and then measuring your blood sugar afterward, because pregnancy hormones can make it harder for your body to use insulin effectively. Left unmanaged, gestational diabetes can raise the chance of a larger-than-average baby, complications during delivery, and higher blood sugar for you later in life, which is part of why screening is offered to almost everyone carrying a pregnancy.
Most pregnant people are offered this test regardless of risk factors, because gestational diabetes often causes no noticeable symptoms at all. Some groups are considered at higher chance, including those over 35, those with a family history of diabetes, those who had gestational diabetes in a previous pregnancy, and those with a higher pre-pregnancy body weight — but plenty of people with none of these factors still test positive. That's exactly why it's a universal screening test rather than one offered only to people who seem likely to have it. A positive result reflects how your body is responding to pregnancy, not anything you did or didn't do.
When is the glucose tolerance test done?
Most people have the glucose tolerance test between 24 and 28 weeks of pregnancy, when the hormonal changes that can affect insulin sensitivity are typically most pronounced. If you have risk factors such as a previous diagnosis of gestational diabetes, obesity, or a strong family history of diabetes, your provider may test you earlier, sometimes at your first prenatal visit, and then test again later in the 24-to-28-week window if the early result comes back normal. Testing twice isn't a sign anything is wrong — it's simply how care plans account for the fact that gestational diabetes can develop at different points across pregnancy.
If you're diagnosed with diabetes before pregnancy or very early on, that's usually managed as pre-existing diabetes rather than the gestational form, and your care team will likely set up a different monitoring plan from the start. If your 24-to-28-week appointment falls slightly outside that window because of scheduling, that's generally not a concern — a week or two either way doesn't meaningfully change what the test can tell you. Your midwife or doctor will let you know if timing needs to be adjusted for any reason specific to your pregnancy.
What's the difference between the one-step and two-step test?
The one-step and two-step approaches are two accepted ways of screening for gestational diabetes, and which one you're offered depends on your clinic or country rather than anything about your individual pregnancy. The one-step test involves drinking a 75-gram glucose solution after fasting, with blood drawn once before and then at one and two hours afterward. The two-step approach starts with a shorter screening test — a 50-gram drink with no fasting required — and only moves to a longer three-hour, 100-gram test if that initial screen comes back elevated. Both are widely used; neither is more accurate in every situation, and professional guidelines differ on which they favor.
Here's how the two approaches compare: | | One-step test | Two-step test | |---|---|---| | Fasting required | Yes | No | | First drink | 75g glucose | 50g glucose | | Blood draws | Fasting, 1hr, 2hr | 1hr (screen); 3 more draws over 3hrs only if screen is elevated | | Follow-up needed | No, single visit | Only if the initial screen is elevated | Neither approach is inherently better for every pregnancy; your clinic's protocol is chosen based on their standard practice, not on anything specific to you. If you're unsure which version you'll be given, it's worth asking ahead so you can prepare correctly.
What do the results mean?
A result is considered positive for gestational diabetes when one or more of your blood sugar readings comes back higher than the threshold your lab uses, and the exact numbers depend on which version of the test you took. For the one-step test, thresholds are typically around 92, 180, and 153 mg/dL for the fasting, one-hour, and two-hour draws; for the two-step test's follow-up three-hour test, thresholds are usually set higher because more glucose is given overall. A single elevated value on the two-step screening test often just means you'll be asked to take the longer diagnostic test next, not that you have gestational diabetes.
If your results confirm gestational diabetes, your provider will talk you through what it means for the rest of your pregnancy — this usually includes monitoring your blood sugar at home, adjusting how and when you eat, and sometimes medication if those changes aren't enough on their own. A diagnosis doesn't mean you did anything wrong; it reflects how your body is responding to pregnancy hormones, which isn't within your control. Most people who are diagnosed go on to have healthy pregnancies and healthy babies with the right monitoring and support from their care team.
How should I prepare for the test?
Preparation depends on which version of the test your clinic uses, so it's worth confirming instructions with your provider ahead of time rather than assuming. For the one-step 75-gram test, you'll typically be asked to fast for 8 to 14 hours beforehand, meaning no food or drink other than water, so many people schedule it for first thing in the morning. For the two-step 50-gram screening test, fasting usually isn't required, and you can eat normally beforehand. Either way, plan to be at the clinic for one to three hours depending on which test you're having, and bring something to occupy your time between draws.
The glucose drink itself is very sweet — often compared to flat soda — and some people feel briefly lightheaded, nauseated, or shaky afterward, especially during the longer three-hour version. Eating a normal meal the evening before, rather than restricting food in anticipation, generally makes the fasting period and the test itself more comfortable. If you feel faint, very unwell, or unable to keep the drink down during the test, tell the staff right away rather than trying to push through it. Arrange a ride home in advance if you think you might feel too unsteady to drive afterward.
What if I can't tolerate the drink or the test?
Some people find the standard glucose drink difficult to keep down because of nausea, its strong sweetness, or an existing condition like reflux that the sugar load aggravates, and there are alternatives your provider can offer. Options include a different flavored glucose beverage, jelly beans standardized to a specific glucose amount, or in some cases a different testing schedule altogether. Vomiting the drink partway through usually means the test needs to be rescheduled, because an incomplete dose can't be interpreted reliably. This isn't a failure on your part — it's simply information your care team needs in order to plan around your particular situation.
If you have a history of severe nausea in pregnancy, gastric bypass surgery, or dumping syndrome, tell your provider before the test is scheduled, since the standard glucose load isn't always appropriate for these situations. Call your provider the same day if you feel severely unwell during or after the test — persistent vomiting, fainting, or chest pain are not expected reactions and should be checked promptly rather than waited out. For most people, though, the test is uncomfortable rather than genuinely difficult, and any lightheadedness or queasiness resolves once you're able to eat and rest afterward.