Gestational Diabetes Test: Screening, OGTT Results, and What to Expect

Gestational diabetes usually causes no obvious symptoms, so blood testing is an important part of prenatal care. Most people who were not already found to have diabetes are screened between 24 and 28 weeks of pregnancy. Earlier testing may be recommended when there is a previous history of gestational diabetes, obesity, polycystic ovary syndrome, a strong family history, an earlier large baby, or another reason to suspect preexisting diabetes or high-risk abnormal glucose metabolism.

When Is the Gestational Diabetes Test Performed?

For most pregnancies without previously identified diabetes, screening is performed at 24–28 weeks. Insulin resistance normally increases during the second and third trimesters because placental hormones change how the body responds to insulin. This is why a normal early result does not always replace routine testing later in pregnancy.

Testing may occur earlier when the patient:

• had gestational diabetes in a previous pregnancy;
• has overweight or obesity plus additional diabetes risk factors;
• has polycystic ovary syndrome;
• has a parent or sibling with type 2 diabetes;
• previously delivered a baby weighing more than 9 pounds or 4.1 kg;
• has prediabetes, abnormal glucose, or metabolic liver disease;
• has signs of insulin resistance such as acanthosis nigricans;
• uses a medicine that can raise glucose;
• has symptoms or laboratory findings suggesting preexisting diabetes.

Risk-factor lists and screening policies vary by country. Some health systems screen all pregnant patients; others use risk-based early testing followed by routine testing at 24–28 weeks.

What Is Early-Pregnancy Diabetes Testing?

Testing early in pregnancy has two related goals:

1. Identify previously unrecognized type 1 or type 2 diabetes. Glucose in the usual nonpregnancy diabetes range early in pregnancy may represent diabetes that existed before pregnancy rather than ordinary gestational diabetes.
2. Identify high-risk abnormal glucose metabolism. ADA 2026 recommends assessment before 15 weeks because some early abnormalities predict later gestational diabetes and adverse pregnancy outcomes.

The clinician may use fasting plasma glucose, HbA1c, random plasma glucose, or an early OGTT depending on the health system and clinical situation.

HbA1c is not the preferred stand-alone test for diagnosing gestational diabetes at 24–28 weeks. Pregnancy changes red-blood-cell turnover, and HbA1c does not show the same information as a glucose challenge or OGTT.

A normal early test may still need to be repeated at 24–28 weeks because pregnancy-related insulin resistance increases later.

Two-Step Testing: 50-g Screening Followed by a 100-g OGTT

This approach is commonly used in the United States.

Step 1: The 50-g glucose challenge test

The glucose challenge test is a screening test, not the final diagnostic test.

• It is usually performed without fasting.
• You drink a solution containing 50 grams of glucose.
• Blood is drawn one hour later.
• You should remain in the testing area and avoid food, smoking, and unplanned activity during the waiting period.

Clinics use different positive-screen thresholds, commonly 130, 135, or 140 mg/dL at one hour. A lower cutoff detects more possible cases but also sends more people for the diagnostic test.

A result above the clinic’s threshold means that a diagnostic OGTT is usually needed. It does not automatically mean that you have gestational diabetes.

Step 2: The fasting 100-g, three-hour OGTT

After an overnight fast, blood is drawn before the drink and at one, two, and three hours after consuming 100 grams of glucose.

The commonly used Carpenter-Coustan thresholds are:

Under the traditional two-step method, gestational diabetes is generally diagnosed when two or more values meet or exceed the thresholds. One abnormal result may still be associated with increased pregnancy risk and may lead to nutrition counseling, repeat testing, home monitoring, or closer follow-up depending on the clinician.

One-Step Testing: The Fasting 75-g, Two-Hour OGTT

The one-step strategy skips the separate 50-g screening test. After an overnight fast:

1. A fasting blood sample is drawn.
2. You drink a solution containing 75 grams of glucose.
3. Blood is drawn one hour later.
4. Blood is drawn again two hours after the drink.

The commonly used IADPSG/ADA/WHO thresholds are:

With this approach, one or more values meeting or exceeding the threshold establishes the diagnosis.

Why Do Gestational Diabetes Thresholds Differ?

There is no single worldwide testing strategy. Different professional organizations and health systems may use:

• a two-step 50-g screen followed by a 100-g OGTT;
• a one-step 75-g OGTT with fasting, one-hour, and two-hour measurements;
• a 75-g OGTT using only fasting and two-hour values;
• different screening cutoffs or country-specific diagnostic thresholds.

The United Kingdom, for example, commonly uses a 75-g OGTT with different diagnostic criteria from the IADPSG/ADA one-step table.

The general adult “normal range” of 65–105 mg/dL should not be used to interpret all pregnancy test results. Each test has a specific glucose load, timing, and threshold.

Are Fasting, Random, and After-Meal Tests Diagnostic for Gestational Diabetes?

They may identify concerning hyperglycemia, but they are not interchangeable with a standardized gestational diabetes protocol.

Fasting plasma glucose

A fasting value may help identify preexisting diabetes or abnormal early-pregnancy glucose. It may also be one component of a 75-g or 100-g OGTT. A single fasting value does not replace the complete diagnostic test when the maternity protocol requires an OGTT.

Random plasma glucose

A random test may be useful when symptoms or severe hyperglycemia are present. “Random” means the sample is taken without regard to the last meal; it does not mean blood is routinely drawn three times before, during, and after meals.

Usual post-meal glucose

A glucose level two hours after an ordinary meal is useful for monitoring after diagnosis, but the meal does not contain a standardized amount of carbohydrate. It is not the same as an OGTT and should not be called the “second best” diagnostic test.

Urine glucose

Glucose in urine is not accurate enough to diagnose or rule out gestational diabetes. Pregnancy changes the kidney threshold for glucose, so glycosuria can occur without gestational diabetes and gestational diabetes can occur without glycosuria.

How Should You Prepare for the Test?

For the 50-g screening test

Fasting is usually not required, but follow your clinic’s written instructions. Some clinics advise avoiding a very sugary meal immediately before testing.

For the 75-g or 100-g diagnostic OGTT

• Follow the specified fasting period, commonly at least eight hours.
• Water is usually allowed, but confirm with the laboratory.
• Do not smoke, vape, chew gum, or consume coffee, tea, juice, milk, sweets, or caloric drinks during the fast.
• Do not stop prescribed medicines unless the maternity team instructs you.
• Tell the clinic about corticosteroids, beta-agonists, illness, vomiting, bed rest, bariatric surgery, or medicines that may affect the result.
• Eat normally in the days before testing unless the clinic provides a specific preparation plan.
• Do not deliberately restrict carbohydrate to try to obtain a lower result.
• Avoid unusual strenuous exercise immediately before or during the test.

The old instruction to consume at least 150 grams of carbohydrate for three days reflects older OGTT-preparation protocols. Some laboratories still provide carbohydrate guidance, while others advise the patient simply to maintain a normal unrestricted diet. Follow your laboratory’s protocol rather than a universal online rule.

What Happens During the OGTT?

Plan to remain at the laboratory for the entire testing period.

1. The staff confirms your fasting status and collects the first sample.
2. You drink the glucose solution within the instructed time.
3. The clock begins according to the laboratory protocol.
4. You remain seated or resting and do not eat, smoke, or exercise.
5. Blood samples are collected at the required times.

Tell the staff immediately if you vomit, feel faint, develop severe nausea, or cannot finish the drink. Vomiting may invalidate the test, but do not decide this yourself or leave without speaking to the staff.

Bring reading material, wear clothing that allows easy blood draws, and consider having someone accompany you if you have a history of fainting or severe nausea.

Can the Glucose Drink Cause Side Effects?

Temporary symptoms may include:

• nausea;
• abdominal fullness;
• headache;
• dizziness or lightheadedness;
• sweating or shakiness;
• fatigue;
• vomiting;
• feeling unwell after the test as glucose falls.

Most symptoms resolve after the test and a permitted meal. Ask the clinic when you may eat, drink, drive, and take morning medicines.

A standard glucose drink does not cause gestational diabetes. It temporarily challenges the body with a measured glucose amount so the response can be assessed.

What If You Had Bariatric Surgery or Cannot Tolerate the Drink?

After gastric bypass or another procedure associated with dumping syndrome, a standard glucose drink may cause severe symptoms and unreliable results. Some patients also cannot complete an OGTT because of recurrent vomiting or another medical condition.

There is no single universal substitute. The obstetric and diabetes teams may consider structured fasting and post-meal home glucose monitoring, continuous glucose monitoring, or another locally approved strategy. Do not substitute home readings without a documented clinical plan.

What Does an Abnormal Result Mean?

An abnormal diagnostic result means that pregnancy-related insulin resistance has exceeded the pancreas’s ability to maintain glucose below the selected thresholds.

It does not mean that:

• you caused the condition by eating sugar;
• you had diabetes before pregnancy;
• the baby will definitely have a complication;
• insulin will always be required;
• diabetes will definitely continue after delivery.

It does mean that treatment and monitoring should begin promptly because managing glucose reduces pregnancy risks.

Possible risks of untreated or insufficiently controlled gestational diabetes include:

• large-for-gestational-age growth and birth injury;
• preeclampsia and high blood pressure;
• polyhydramnios;
• preterm delivery;
• cesarean delivery;
• newborn low blood glucose;
• newborn breathing problems;
• higher future risk of obesity and type 2 diabetes for the child;
• higher future type 2 diabetes risk for the mother.

Most people with gestational diabetes can have a healthy pregnancy and baby with appropriate care.

Common Glucose Targets After Gestational Diabetes Is Diagnosed

These are treatment targets, not OGTT diagnostic thresholds.

Your maternity team may use different goals based on the pregnancy, glucose method, medication, fetal growth, and hypoglycemia risk.

A reading of 140 mg/dL has a different meaning when it is:

• one hour after the 50-g screening drink;
• three hours into a 100-g OGTT;
• one hour after an ordinary meal;
• two hours after an ordinary meal.

Always record the timing.

What Happens After Diagnosis?

Care commonly includes:

• instruction on home glucose monitoring;
• an individualized pregnancy meal plan;
• appropriate carbohydrate distribution rather than eliminating carbohydrate;
• physical activity such as walking after meals when obstetrically safe;
• weight-gain guidance for pregnancy—not active weight-loss dieting;
• fetal growth and pregnancy monitoring;
• medicine when glucose remains above target.

Insulin is the preferred medicine in many guidelines when lifestyle measures are insufficient because it does not cross the placenta. Metformin is used in some health systems after individualized discussion, but it crosses the placenta and is not appropriate for every patient.

Do not start supplements, herbs, a ketogenic diet, prolonged fasting, or severe carbohydrate restriction to lower pregnancy glucose.

Read Gestational Diabetes Diet and Treatment Strategies and Tips for Controlling Gestational Diabetes.

Do You Need Testing After the Baby Is Born?

Yes. Glucose often improves after delivery when placental hormones fall, but this should be confirmed.

ADA 2026 recommends:

• a 75-g OGTT at 4–12 weeks postpartum using nonpregnancy diagnostic criteria;
• lifelong screening for prediabetes or type 2 diabetes every 1–3 years afterward.

The postpartum OGTT is preferred over HbA1c alone during the early postpartum period because blood loss, pregnancy-related changes in red-cell turnover, and iron status can affect HbA1c.

A history of gestational diabetes also increases the chance of recurrence in a later pregnancy. Tell future maternity teams early, even when postpartum glucose was normal.

When Should You Contact the Maternity Team Urgently?

Contact the maternity or diabetes team promptly for:

• repeated readings above your pregnancy targets;
• repeated low glucose;
• vomiting or difficulty keeping fluids down;
• illness, fever, or infection;
• ketones;
• severe thirst, frequent urination, or unexplained weight loss;
• a medication error or missed insulin;
• reduced fetal movement, bleeding, contractions, or fluid leakage.

Common Testing Mistakes to Avoid

• Assuming the 50-g screen is diagnostic.
• Using 75-g thresholds to interpret a 100-g test.
• Comparing an ordinary meal reading with an OGTT threshold.
• Eating or drinking calories during a required fast.
• Walking around or exercising during the waiting period.
• Leaving before all timed samples are collected.
• Restricting carbohydrate for several days to try to change the result.
• Using a home meter instead of laboratory plasma glucose for diagnosis.
• Assuming a normal early test means the 24–28-week test is unnecessary.
• Skipping postpartum testing because glucose improved after delivery.

Most Asked Questions

Do I need to fast for the one-hour glucose test?

The 50-g screening test is usually nonfasting. Follow your clinic’s instructions because local protocols differ.

What is a normal result on the one-hour screening test?

The clinic may use a cutoff of 130, 135, or 140 mg/dL. A value below its selected threshold usually means no diagnostic OGTT is needed unless there is another clinical concern.

Does failing the one-hour test mean I have gestational diabetes?

No. It usually means the diagnostic 100-g OGTT is needed. Some clinics use a one-step 75-g strategy instead.

Can I drink water during the fasting OGTT?

Water is commonly permitted, but confirm with the laboratory. Do not drink flavored, sweetened, or caloric beverages.

Can I walk around during the test?

Usually no. Physical activity can change glucose handling. Remain seated or resting unless staff instructs otherwise.

What happens if I vomit the glucose drink?

Tell the staff immediately. The test may need to be stopped and repeated or replaced with another clinically approved strategy.

Can HbA1c replace the gestational diabetes test?

Not routinely at 24–28 weeks. HbA1c does not provide the same diagnostic information as a pregnancy OGTT.

Can gestational diabetes occur after a normal test?

Yes, although it is less common. Contact the maternity team if later fetal growth, symptoms, glucose readings, medicines, or another clinical finding raises concern.

Will gestational diabetes disappear after delivery?

Glucose often returns to normal, but not always. A postpartum 75-g OGTT is required to check for persistent diabetes or prediabetes.

Related Questions

• Is Rapid Weight Gain a Sign of Gestational Diabetes?
• Can I Prevent Gestational Diabetes Again in This Pregnancy?
• How Should Different Fasting and Two-Hour Glucose Results Be Interpreted?

Related Resources

• Signs of Gestational Diabetes
• Gestational Diabetes Diet and Treatment Strategies
• Controlling Gestational Diabetes
• Blood Tests for Diabetes
• Blood Sugar Levels After Eating
• Blood Sugar Log Sheet

Medical disclaimer: This page provides general education and cannot select your pregnancy screening method, diagnose gestational diabetes, or replace your obstetric or diabetes team. Follow the exact preparation instructions and diagnostic thresholds supplied by your maternity clinic and laboratory.

References

1. American Diabetes Association: Diagnosis and Classification of Diabetes—Standards of Care in Diabetes 2026
2. American Diabetes Association: Management of Diabetes in Pregnancy—Standards of Care in Diabetes 2026
3. American College of Obstetricians and Gynecologists: Gestational Diabetes
4. National Institute of Diabetes and Digestive and Kidney Diseases: Gestational Diabetes
5. Centers for Disease Control and Prevention: Gestational Diabetes
6. U.S. Preventive Services Task Force: Gestational Diabetes Screening
7. NICE: Diabetes in Pregnancy—Gestational Diabetes Testing and Management
8. World Health Organization: Diagnostic Criteria and Classification of Hyperglycaemia First Detected in Pregnancy