Do I need to fast for gestational diabetes screening (GDS) in the 2nd trimester?

Gestational Diabetes Screening Instructions for Second Trimester

Fasting Requirements

For the standard two-step screening approach, you do NOT need to fast for the initial 50-gram glucose challenge test (GCT), but you WILL need to fast if you screen positive and require the follow-up diagnostic test. 1

Two-Step Approach (Most Common in U.S.)

Step 1 - Initial Screening (NO FASTING REQUIRED):

• You will drink a 50-gram glucose solution at any time of day, regardless of when you last ate 1
• Blood will be drawn exactly 1 hour after drinking the glucose solution 1
• This screening is performed between 24-28 weeks of pregnancy 1
• If your 1-hour blood glucose is ≥140 mg/dL (some centers use 130 or 135 mg/dL), you will need the diagnostic test 1

Step 2 - Diagnostic Test (FASTING REQUIRED if Step 1 is abnormal):

• You must fast for at least 8 hours overnight before this test 1
• You will have a fasting blood draw, then drink a 100-gram glucose solution 1
• Blood will be drawn at 1,2, and 3 hours after the glucose drink 1
• Gestational diabetes is diagnosed if 2 or more values meet or exceed: fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, 3-hour ≥140 mg/dL 1

One-Step Approach (Alternative Method)

If your provider uses the one-step approach, you WILL need to fast:

• Fast for at least 8 hours overnight 1
• You will have a fasting blood draw, then drink a 75-gram glucose solution 1
• Blood will be drawn at 1 and 2 hours after the glucose drink 1
• Gestational diabetes is diagnosed if ANY single value meets or exceeds: fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, or 2-hour ≥153 mg/dL 1, 2

What to Expect During Testing

Physical Experience:

• The glucose drink is very sweet, similar to flat soda, and may cause mild nausea 2
• You must remain seated and avoid smoking during the test 1
• You should not eat, drink (except water), or exercise during the testing period 1
• Most women tolerate the test well, though some experience temporary nausea or lightheadedness 2

Timing Considerations:

• The initial screening test takes approximately 1 hour for the two-step approach 1
• The diagnostic test (if needed) takes 3 hours for the two-step approach or 2 hours for the one-step approach 1
• Schedule the test in the morning when possible, especially if fasting is required 1

If You Have Risk Factors

You may need earlier screening at your first prenatal visit if you have: 1, 2

• BMI ≥30 kg/m² (obesity) 2
• Previous history of gestational diabetes 1, 2
• Family history of diabetes in a first-degree relative 1
• Previous baby weighing >4500 grams (9.9 pounds) 1
• Belong to high-risk ethnic groups (Hispanic, Native American, South/East Asian, African American, Pacific Islander) 1, 2
• Polycystic ovary syndrome 2
• Glucose detected in your urine 1

If early screening is negative, you will still need repeat screening at 24-28 weeks. 2, 3

Common Pitfalls to Avoid

• Do not assume you can skip the 24-28 week screening even if you had normal early testing - glucose tolerance naturally worsens as pregnancy progresses 1, 2
• Do not eat or drink anything except water once fasting begins for the diagnostic tests - this will invalidate results 1
• Do not schedule the test when you are ill - acute illness can affect glucose levels 1
• Confirm with your specific provider which approach they use (one-step vs. two-step) to know whether initial fasting is required 1

Why This Testing Matters

Screening and treating gestational diabetes reduces serious complications: 1

• Decreases risk of preeclampsia (dangerous high blood pressure in pregnancy) 1
• Reduces risk of having a very large baby (macrosomia), which can complicate delivery 1
• Lowers risk of shoulder dystocia (baby's shoulder getting stuck during delivery) 1
• 80-90% of women diagnosed with mild gestational diabetes can be managed with diet and lifestyle changes alone, without medication 1, 2