Gestational diabetes (GDM) is glucose intolerance first identified in pregnancy. It affects roughly 6 to 9 percent of pregnancies in the United States, with rates rising along with background obesity and average maternal age. Diagnosed and managed, most pregnancies with GDM have outcomes very close to non-diabetic pregnancies. Undiagnosed, GDM raises risks of macrosomia, shoulder dystocia, neonatal hypoglycemia, pre-eclampsia, and cesarean delivery.
Screening is universal in the US. Below is what to expect, what the two main testing approaches look like, and what the diagnostic numbers actually mean.
When screening happens
ACOG Practice Bulletin 190 (2018, reaffirmed) recommends:
- Universal screening at 24 to 28 weeks for everyone without pre-existing diabetes.
- Early screening at the first prenatal visit for anyone with risk factors.
Risk factors that trigger early screening:
- BMI 30 or higher (or 25 in higher-risk ethnic groups).
- Prior GDM.
- Prior macrosomic infant (4,000 grams or higher).
- First-degree relative with type 2 diabetes.
- PCOS.
- Known glucose intolerance or borderline labs.
- Habitual stillbirth or other prior pregnancy loss without identified cause.
For more on PCOS as a risk factor, see PCOS and fertility and our post on insulin resistance.
One-step vs two-step: the long debate
There are two main approaches in current use, and US practice varies.
Two-step (Carpenter-Coustan), the long-standing US standard
- 50-gram glucose challenge test (GCT). Non-fasting. Drink 50g glucose, blood draw at 1 hour. Threshold typically 140 mg/dL (some institutions use 130 or 135). About 15 to 20 percent of people screen positive.
- 100-gram 3-hour oral glucose tolerance test (OGTT). Fasting. Blood draws fasting, 1, 2, and 3 hours after the 100g drink. GDM is diagnosed if 2 or more of the 4 values exceed thresholds (typically 95, 180, 155, 140 mg/dL).
Roughly 2 to 3 percent of all screened people end up with a GDM diagnosis using two-step.
One-step (IADPSG / WHO), the international approach
A single 75-gram 2-hour OGTT, fasting. Diagnosis if any one of:
- Fasting 92 mg/dL or higher.
- 1-hour 180 mg/dL or higher.
- 2-hour 153 mg/dL or higher.
One-step diagnoses about twice as many people as two-step (roughly 6 to 9 percent).
Which is better?
Hillier et al. (NEJM 2021) directly compared the two strategies in a randomized trial (ScreenR2GDM) of over 23,000 pregnant people. The result: one-step doubled the GDM diagnosis rate but did not significantly improve perinatal outcomes (large-for-gestational-age, primary cesarean, hypertensive disorders, neonatal hypoglycemia, perinatal composite).
This supported ACOG’s continued recommendation that either approach is acceptable, with two-step remaining the most common US standard. The trial did not settle the broader question — there is ongoing debate about whether the one-step approach catches mild cases that benefit from earlier intervention even if trial-level outcomes are similar.
What you experience depends on your hospital’s chosen pathway.
What the numbers mean
If your 1-hour screen comes back at 145, the next step is the 3-hour test, not a diagnosis. About 1 in 4 people who screen positive on the 1-hour go on to fail the 3-hour. The remaining 3 in 4 have a normal 3-hour and no GDM.
If your 1-hour is dramatically elevated (often above 200), some institutions skip directly to diagnosis or proceed straight to A1C and management — confirm with your provider.
A normal screen reduces but does not eliminate GDM probability. If you develop concerning findings later (excessive fetal growth, polyhydramnios), retesting later in pregnancy is reasonable.
What treatment looks like
Mild GDM is managed with:
- Diet changes. Carbohydrate counting (typically 175 to 200 grams/day distributed across 3 meals and 2 to 3 snacks), avoiding spikes from refined carbs, increased protein and fiber.
- Glucose monitoring. Fasting and 1 or 2 hours after each meal, 4 to 5 times daily. Targets typically fasting under 95 mg/dL and 1-hour postprandial under 140 mg/dL.
- Walking or other movement. Even 10 to 15 minutes of walking after meals lowers postprandial glucose meaningfully.
If diet and movement do not bring numbers into target after 1 to 2 weeks, medication is added. Insulin is the first-line in pregnancy. Metformin is sometimes used and is considered acceptable in current ACOG guidance, though insulin remains the gold standard. Glyburide use has fallen out of favor.
What GDM means for delivery
- Timing. Well-controlled GDM on diet alone is typically delivered by 41 weeks. GDM on medication is usually delivered by 39 to 40 weeks. Poorly controlled GDM may be delivered earlier.
- Mode. GDM by itself is not an indication for cesarean. Estimated fetal weight 4,500 grams or higher in GDM is sometimes treated as a relative indication for primary cesarean per ACOG.
- Monitoring. Increased ultrasound surveillance for fetal growth, often with biophysical profile testing in the third trimester.
- Newborn glucose checks. Babies of GDM mothers are checked for hypoglycemia in the first hours after birth. Most do not need any intervention.
After delivery
- GDM typically resolves within hours of delivery, but glucose follow-up is important.
- ACOG and ADA recommend a 75-gram 2-hour OGTT at 4 to 12 weeks postpartum to detect undiagnosed type 2 diabetes or persistent glucose intolerance.
- Lifelong diabetes risk after GDM is roughly 50 percent over 10 years. This is the strongest modifiable signal in many people’s records, and breast feeding, weight management, and movement all measurably lower risk.
- Future pregnancies have higher GDM recurrence risk (around 40 to 60 percent), so early screening is recommended.
Where this fits in your timeline
GDM screening lands in the same window as anatomy ultrasound and the start of third-trimester surveillance. The Pregnancy Week Calculator helps you track when 24 to 28 weeks lands relative to your due date.
Questions worth asking
- Does your practice use one-step or two-step?
- Am I a candidate for early screening?
- If I screen positive, when will I know the result and the next steps?
- What dietitian and diabetes educator support is available?
- If I need medication, what is your default?
The bottom line
GDM screening is universal at 24 to 28 weeks, with earlier testing for higher-risk people. The 50-gram 1-hour screen plus the 3-hour confirmatory test (two-step) remains the US standard, but the 75-gram one-step is also accepted. Hillier et al. NEJM 2021 found similar outcomes between strategies despite different diagnosis rates. Most GDM is managed with diet and movement; some needs insulin. Postpartum follow-up testing matters for long-term health and is too often skipped. Knowing what the test is and what the numbers mean keeps the diagnosis from feeling like a black box.