Glucose Test During Pregnancy: What It Tests, When It Is Done, and How to Prepare

Gestational diabetes (GDM) affects approximately 6–9% of pregnancies in the United States. It develops when pregnancy hormones block insulin's ability to lower blood sugar, causing glucose to build up in the bloodstream. Because GDM typically causes no symptoms, glucose testing is the only reliable way to detect it — and detecting it early significantly reduces the risk of complications for both mother and baby.

The two-step glucose testing process

Most U.S. obstetricians follow the two-step approach recommended by the American College of Obstetricians and Gynecologists (ACOG):

Step 1 — Glucose Challenge Test (GCT), also called the 1-hour screen:

• Performed between 24 and 28 weeks of pregnancy (or earlier if risk factors are present)
• No fasting required — you can eat normally beforehand
• Drink a 50-gram glucose solution (a sweet syrup provided at the lab)
• Blood is drawn exactly one hour later
• Result below 130 mg/dL (some providers use 140 mg/dL): screen negative, no further testing needed
• Result at or above the cutoff: proceed to Step 2

Step 2 — Oral Glucose Tolerance Test (OGTT), also called the 3-hour test:

• Requires an 8-hour overnight fast before the test
• A fasting blood draw is taken on arrival
• Drink a 100-gram glucose solution
• Blood is drawn at 1 hour, 2 hours, and 3 hours after the drink
• GDM is diagnosed if two or more of the four values meet or exceed the thresholds below (Carpenter-Coustan criteria, used by most U.S. labs):

Who is tested earlier than 24 weeks?

ACOG recommends screening at the first prenatal visit for patients with significant risk factors for undiagnosed Type 2 diabetes. Early first-trimester screening uses standard diabetes diagnostic criteria (fasting glucose ≥126 mg/dL, or HbA1c ≥6.5%), not the GDM cutoffs.

Risk factors that prompt earlier screening include:

• BMI above 25 (or above 23 in Asian American patients)
• Prior history of GDM in a previous pregnancy
• Prior delivery of a baby weighing more than 9 pounds (macrosomia)
• Polycystic ovarian syndrome (PCOS)
• Family history of Type 2 diabetes in a first-degree relative
• Acanthosis nigricans (a skin sign of insulin resistance)
• Hemoglobin A1c of 5.7–6.4% (prediabetes range) before pregnancy

What does a GDM diagnosis mean for your pregnancy?

GDM is manageable and most women with it go on to have healthy pregnancies and babies — but it requires active monitoring and treatment to keep blood sugar in the target range throughout the third trimester.

Blood sugar monitoring: Most providers prescribe a home glucose meter to check fasting and post-meal glucose levels daily. Target ranges are typically: fasting below 95 mg/dL, one hour after meals below 140 mg/dL, two hours after meals below 120 mg/dL.

Dietary management: A registered dietitian trained in GDM will develop a carbohydrate-controlled meal plan. Carbohydrates are spread evenly through the day to prevent glucose spikes; most women do not need to eliminate carbohydrates entirely, but timing and portion control matter significantly.

Medication: If diet and exercise alone are insufficient to keep glucose in range after 1–2 weeks, insulin or oral medications (metformin or glyburide, though both have nuances in pregnancy) are added. Insulin is the first-line medication recommended by ACOG because it does not cross the placenta.

Fetal monitoring: Women with GDM typically receive additional ultrasounds (to monitor for fetal growth acceleration/macrosomia) and may begin non-stress tests (NSTs) in the third trimester. Delivery timing is often individualized based on glucose control and fetal size.

Risks of unmanaged gestational diabetes

When GDM goes undetected or poorly controlled, the elevated glucose passes through the placenta to the baby, causing the baby's pancreas to overproduce insulin. This creates several risks:

• Macrosomia (large baby, birth weight above 9 lbs) — increases the risk of shoulder dystocia during delivery and cesarean delivery
• Neonatal hypoglycemia — after delivery, the baby's pancreas continues overproducing insulin without the maternal glucose supply, causing blood sugar to drop sharply in the first hours of life
• Preterm birth — elevated glucose increases inflammation and can trigger early labor
• Preeclampsia — GDM raises the risk of high blood pressure disorders of pregnancy
• Stillbirth — rare, but risk is elevated with poorly controlled GDM, particularly in the third trimester

What happens after delivery?

GDM resolves in the vast majority of women after delivery — but it is a significant risk marker for future Type 2 diabetes. ACOG recommends:

• A 75-gram OGTT (two-hour, non-pregnant test) at 4–12 weeks postpartum to confirm glucose has returned to normal
• Annual fasting glucose or HbA1c testing long-term — women with a history of GDM have a 50% lifetime risk of developing Type 2 diabetes
• Lifestyle counseling: breastfeeding (modestly reduces T2D risk), weight management, and regular physical activity reduce conversion to diabetes

How to prepare for the glucose challenge test (GCT)

• No fasting required for the 1-hour GCT — eat your normal meals and snacks
• Avoid an unusually high-carbohydrate meal immediately before the test (a large fast-food meal 20 minutes prior may nudge borderline results, though this is contested in the literature)
• Plan to stay at the collection site for one hour after drinking the glucose solution
• Bring a snack for after the draw — many women feel shaky or nauseated after the glucose drink on an empty or nearly-empty stomach

For the 3-hour OGTT: fast completely from midnight the night before (water is fine). Plan to be at the collection site for 3–3.5 hours. Bring something to occupy the wait. Do not eat, exercise, or smoke during the test period, as any of these can affect glucose readings.

Sources and further reading

• MedlinePlus: Glucose Testing
• ACOG: Gestational Diabetes

Preparing for the glucose challenge test

The one-hour glucose challenge test (GCT) is typically administered non-fasting — you do not need to fast overnight beforehand. However, some providers recommend avoiding high-carbohydrate meals in the 1–2 hours before the test, as very recent carbohydrate intake can push borderline results into the abnormal range.

Practical preparation:

• Arrive at your appointment having eaten normally that morning — no fasting unless your provider specifically instructed otherwise
• Avoid high-sugar meals or drinks (juice, soda, candy) in the 2 hours before the test
• Drink the 50g glucose solution within 5 minutes as instructed
• Do not eat, drink (other than water), or exercise during the one-hour wait
• Your blood will be drawn exactly 60 minutes after finishing the glucose drink

What "failed" glucose test results mean — and what comes next

A glucose level at or above 140 mg/dL on the one-hour screen (some providers use 130 mg/dL as the threshold) is considered abnormal and triggers the three-hour diagnostic test. This is not a diagnosis of gestational diabetes — roughly 15–23% of women have an elevated one-hour screen, and only about one-third of those (5–8% overall) are subsequently diagnosed with GDM on the three-hour test.

On the three-hour test, gestational diabetes is diagnosed when two or more of the four blood draws (fasting, 1-hour, 2-hour, 3-hour) meet or exceed the threshold values. Meeting only one threshold is not diagnostic but may prompt closer monitoring or repeat testing.

Gestational diabetes — why early detection matters

GDM that goes undetected or unmanaged significantly increases risk for:

• Macrosomia (large baby) — increasing the risk of delivery complications, shoulder dystocia, and C-section
• Neonatal hypoglycemia — the baby's insulin production overshoots after delivery when the glucose supply from the mother stops
• Preeclampsia — high blood pressure condition of pregnancy; risk is elevated with GDM
• Long-term risk for mother — 50% of women with GDM develop type 2 diabetes within 5–10 years of delivery
• Long-term risk for baby — increased risk of obesity and type 2 diabetes in childhood and adulthood

Well-managed GDM — through diet, exercise, and when needed, insulin or oral medication — significantly reduces these risks and outcomes for most patients are excellent.