What is gestational diabetes in pregnant women, particularly those over 35 years old, with obesity, or a family history of diabetes?

What is Gestational Diabetes?

Gestational diabetes mellitus (GDM) is any degree of glucose intolerance with onset or first recognition during pregnancy, affecting approximately 7% of all pregnancies in the United States, with prevalence ranging from 1-14% depending on the population studied. 1

Core Definition and Pathophysiology

• GDM encompasses two distinct scenarios: true hyperglycemia developing at or after 24 weeks due to physiological insulin resistance (approximately 85% of cases), or pre-existing undiagnosed type 2 diabetes or prediabetes that existed before pregnancy but was only discovered during prenatal care 1

• The definition applies regardless of whether insulin or only dietary modification is used for treatment and whether or not the condition persists after pregnancy 1

• This definition does not exclude the possibility that unrecognized glucose intolerance may have preceded or begun simultaneously with the pregnancy 1

High-Risk Populations

Women over 35 years old, those with obesity, or those with a family history of diabetes face substantially elevated risk and require specific screening approaches 1:

• Women with marked obesity (BMI ≥30 kg/m²), personal history of GDM, glycosuria, or strong family history of diabetes should undergo glucose testing as soon as feasible at the first prenatal visit 1, 2

• If initial screening is negative in high-risk women, retesting between 24-28 weeks of gestation is mandatory because GDM typically develops in the second or third trimester due to increasing insulin resistance 1, 2

• Age >35 years, obesity, and family history of diabetes are independent risk factors that significantly increase prevalence from the baseline 1.4-2.8% in low-risk populations to 3.3-6.1% in high-risk groups 1

Screening and Diagnosis

Timing of Screening

• All pregnant women of average or high risk should have testing at 24-28 weeks of gestation 1, 3

• Low-risk status (no screening required) is limited to women meeting ALL of the following: age <25 years, normal pre-pregnancy weight, member of ethnic group with low diabetes prevalence, no known diabetes in first-degree relatives, no history of abnormal glucose tolerance, and no history of poor obstetrical outcomes 1

Diagnostic Approaches

Two acceptable strategies exist 1, 3:

One-step approach:

• Perform 75g oral glucose tolerance test (OGTT) without prior screening 1, 3
• Diagnosis requires ANY ONE of the following plasma glucose values: 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
• May be cost-effective in high-risk populations 1

Two-step approach:

• Initial screening with 50g glucose challenge test (GCT), using threshold of 130-140 mg/dL 1, 3
• If positive, perform diagnostic 100g OGTT on separate day 1, 3
• Diagnosis requires TWO OR MORE of the following plasma glucose values: fasting ≥95 mg/dL (5.3 mmol/L), 1-hour ≥180 mg/dL (10.0 mmol/L), 2-hour ≥155 mg/dL (8.6 mmol/L), 3-hour ≥140 mg/dL (7.8 mmol/L) 1, 3

Critical Testing Requirements

• The OGTT must be performed in the morning after an overnight fast of 8-14 hours and after at least 3 days of unrestricted diet (≥150g carbohydrate per day) and unlimited physical activity 1

• The patient should remain seated and not smoke throughout the test 1

• HbA1c is NOT recommended for GDM screening at 24-28 weeks as it does not perform as well as glucose tolerance testing 2

Maternal and Fetal Complications

Immediate Pregnancy Risks

• Fetal macrosomia (birth weight >90th percentile) shows strong graded association with maternal glycemia, with odds ratio of 7.7 for pre-gestational diabetes 1

• Neonatal hypoglycemia occurs in 10-40% of infants, with higher prevalence in mothers with type 1 diabetes, macrosomic neonates, or prematurity 1

• Maternal complications include increased cesarean delivery rates, preeclampsia, and hypertensive disorders 1, 4

• Perinatal mortality is increased in women with pre-gestational diabetes (OR 3.6 for type 1 diabetes, 1.8 for type 2 diabetes) 1

Long-Term Consequences

• Women with GDM are at substantially increased risk for developing type 2 diabetes after pregnancy, with obesity and insulin resistance enhancing this risk 1

• Offspring of women with GDM face increased risk of obesity, glucose intolerance, and diabetes in late adolescence and young adulthood 1

• The risk of developing type 2 diabetes ranges from 20-50% among women with a history of GDM 5

Management Principles

Glycemic Targets

The American Diabetes Association recommends the following targets for adequate GDM control 6:

• Fasting blood glucose <95 mg/dL (5.3 mmol/L)
• 1-hour postprandial <140 mg/dL (7.8 mmol/L)
• 2-hour postprandial <120 mg/dL (6.7 mmol/L)

Treatment Approach

• Medical nutrition therapy (MNT) is the cornerstone of initial treatment, with individualized meal planning including minimum 175g carbohydrates, 71g protein, and 28g fiber daily 6, 4

• Insulin therapy is indicated when glycemic targets are not met with lifestyle modifications alone, particularly in obese women in the third trimester who experience increased insulin resistance 6, 4

• Insulin is the preferred pharmacological treatment as it does not cross the placenta in measurable amounts, making it safer than oral hypoglycemics 6

• Metformin and glyburide are not recommended as first-line treatments due to their ability to cross the placenta and lack of long-term safety data 6

Postpartum Follow-Up

• Women with GDM should be screened for persistent diabetes at 4-12 weeks postpartum using a 75g OGTT with non-pregnancy diagnostic criteria 2, 3

• Lifelong screening for diabetes or prediabetes at least every 3 years is mandatory 2, 3

• Women found to have prediabetes should receive intensive lifestyle interventions or metformin to prevent progression to diabetes 2, 3

• Breastfeeding may reduce obesity risk in children and is recommended for families 7

Common Pitfalls to Avoid

• Failing to rescreen at 24-28 weeks if early screening is negative leads to missed diagnoses, as GDM typically develops later in pregnancy 2

• Using only age or single risk factors to determine screening necessity misses the requirement that low-risk status demands meeting ALL exclusion criteria simultaneously 1

• Assuming GDM resolution after delivery eliminates future diabetes risk ignores the 20-50% conversion rate to type 2 diabetes 5