Gestational Diabetes Mellitus: Incidence, Screening, Management, and Follow-Up
Incidence and Prevalence
Gestational diabetes mellitus (GDM) affects approximately 7% of all pregnancies in the United States, with a range of 1–14% depending on the population studied and diagnostic criteria used, resulting in more than 200,000 cases annually. 1 The prevalence is rising due to the obesity epidemic, delayed childbearing, and increasing rates of multiple gestations. 2
Universal Screening Recommendations at 24–28 Weeks
All pregnant women without known pre-existing diabetes should undergo universal screening for GDM between 24 and 28 weeks of gestation, the period of maximal pregnancy-related insulin resistance when gestational diabetes most commonly manifests. 1, 3 This timing is critical because diagnostic thresholds were validated specifically during this gestational window based on the HAPO study, which demonstrated continuous associations between maternal glucose levels and adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia, preeclampsia) without a clear risk threshold. 1, 3
Two Screening Approaches
Two-step approach (ACOG-supported):
- Step 1: Administer a 50-g non-fasting glucose challenge test; a 1-hour plasma glucose ≥140 mg/dL (or ≥130 mg/dL for higher sensitivity) triggers diagnostic testing. 1
- Step 2: Perform a 100-g oral glucose tolerance test (OGTT) after an 8–14 hour overnight fast with measurements at fasting, 1,2, and 3 hours. 1
- Diagnosis requires at least two abnormal values (Carpenter-Coustan criteria): fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, 3-hour ≥140 mg/dL. 1
- This approach identifies approximately 5–6% of pregnancies with GDM. 3
One-step approach (IADPSG/ADA-recommended):
- Perform a single 75-g OGTT after an 8–14 hour overnight fast with plasma glucose measured at fasting, 1 hour, and 2 hours. 1, 3
- Diagnosis requires any single abnormal value: fasting ≥92 mg/dL (5.1 mmol/L), 1-hour ≥180 mg/dL (10.0 mmol/L), or 2-hour ≥153 mg/dL (8.5 mmol/L). 1, 3
- This approach identifies approximately 15–20% of pregnancies with GDM, capturing milder hyperglycemia that still benefits from treatment. 3
Test Preparation Requirements
Patients must fast for 8–14 hours before the OGTT and consume at least 150 grams of carbohydrate daily for three days preceding the test to ensure adequate glycogen stores. 1, 3 During the OGTT, patients should remain seated throughout and avoid smoking. 1
Exemptions from Screening
Only women meeting ALL of the following low-risk criteria may skip GDM screening: age <25 years, pre-pregnancy BMI ≤25 kg/m², no first-degree relative with diabetes, no history of abnormal glucose tolerance, no history of poor obstetrical outcomes, and belonging to an ethnic group with low diabetes prevalence. 1 In practice, most pregnant women require screening because these criteria are stringent. 4
First-Line Management: Diet and Exercise
Once GDM is diagnosed, treatment begins with medical nutrition therapy and physical activity as first-line interventions (Evidence level A). 5 The goal is to maintain normoglycemia and prevent excessive gestational weight gain to reduce maternal and fetal complications. 6
Nutritional Therapy
Calorie restriction with a low glycemic index diet is recommended to avoid postprandial hyperglycemia and reduce insulin resistance. 6 All women with GDM should receive nutritional counseling from trained dieticians and be instructed in blood glucose self-monitoring. 5
Exercise Recommendations
Women with GDM should be motivated to increase physical activity to moderate intensity levels (such as walking) if not contraindicated (Evidence level A). 5 Regular physical activity improves insulin sensitivity and helps maintain glucose targets. 6
Glucose Monitoring Targets
Target glucose levels are: fasting <95 mg/dL and 1-hour postprandial <140 mg/dL (Evidence level B). 5 Blood glucose levels, HbA1c, and ketonuria should be monitored to assess the efficacy of lifestyle modifications. 6
Pharmacologic Therapy Options
If blood glucose levels cannot be maintained within therapeutic targets despite lifestyle modifications, pharmacologic treatment should be initiated. 5
Insulin Therapy (First-Line)
Insulin is the recommended first-line pharmacologic treatment for GDM when diet and exercise fail to achieve glucose targets (Evidence level A). 4, 5, 7 Insulin does not cross the placenta and has the longest safety record in pregnancy. 7
Metformin (Alternative)
Metformin has been used more commonly in diabetic pregnant women when insulin cannot be prescribed, after its safety has been proven. 6 However, up to 46% of women on metformin may require additional insulin to maintain expected blood glucose levels. 6 Metformin crosses the placenta, and long-term safety data on offspring are still being collected. 7
Other Oral Antidiabetics
Evidence on the long-term safety of other oral antidiabetic agents during pregnancy is lacking, and they are not routinely recommended. 6
Postpartum Follow-Up Recommendations
All women diagnosed with GDM should be screened for persistent diabetes at 4–12 weeks postpartum using a 75-g OGTT with non-pregnancy diagnostic criteria (not A1C). 1, 4, 5 This is critical because insulin resistance typically resolves after delivery, but reclassification to overt diabetes occurs in a notable proportion of women. 4
Long-Term Surveillance
Women with a history of GDM should have lifelong screening for diabetes or prediabetes at least every 3 years, as they have a 3.4-fold increased risk of developing type 2 diabetes. 4, 2 Coexisting obesity and progressive weight gain are additive factors for progression to type 2 diabetes. 2
Preventive Interventions
Women found to have prediabetes postpartum should receive intensive lifestyle interventions or metformin therapy to prevent progression to overt diabetes (Evidence level A). 1, 4 Continued lifestyle modifications, breastfeeding, and weight management can reduce the risk of future diabetes. 5, 7
High-Risk Women Requiring Early Screening
Women at very high risk should be screened for diabetes as early as possible at the first prenatal visit (12–14 weeks) to detect pre-existing undiagnosed type 2 diabetes. 1, 4 High-risk criteria include:
- BMI ≥30 kg/m² 4
- Prior history of GDM or delivery of a large-for-gestational-age infant 1
- Presence of glycosuria 1
- Diagnosis of polycystic ovary syndrome (PCOS) 1
- Strong family history of type 2 diabetes (first-degree relatives) 1, 4
- High-risk ethnicity (Hispanic, Native American, South/East Asian, African American, Pacific Islander) 4
If early screening is negative in high-risk women, repeat screening at 24–28 weeks is mandatory because insulin resistance rises exponentially in the second and third trimesters. 4, 5
Common Pitfalls to Avoid
- Do not postpone screening beyond 28 weeks; delayed diagnosis increases rates of preeclampsia, macrosomia, shoulder dystocia, and neonatal hypoglycemia. 4
- Do not use hemoglobin A1C for GDM screening; it has poor sensitivity and specificity for detecting gestational diabetes. 3, 5
- Do not rely on urine glucose testing for GDM detection or management, as the renal glucose threshold decreases during pregnancy. 3, 4
- Do not skip repeat screening at 24–28 weeks in high-risk women who had a negative early screen. 4
- Do not apply gestational diabetes diagnostic thresholds (92/180/153 mg/dL) to early-pregnancy screening; use non-pregnancy criteria (fasting ≥126 mg/dL) to detect overt diabetes. 3, 4