Gestational diabetes

Definition

Gestational diabetes mellitus is a form of glucose intolerance first recognized during pregnancy. It is characterized by hyperglycaemia of variable severity with onset or first detection during gestation, usually in the second or third trimester. It results from a combination of increased insulin resistance and inadequate compensatory insulin secretion.

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Pathophysiology

• Pregnancy-induced insulin resistance occurs due to placental hormones such as human placental lactogen (hPL), progesterone, cortisol, growth hormone, and prolactin.

• In most pregnancies, pancreatic β-cells compensate by increasing insulin secretion.

• In GDM, this compensation is insufficient, leading to maternal hyperglycaemia.

• Hyperglycaemia crosses the placenta, causing fetal hyperinsulinaemia, which contributes to macrosomia and neonatal complications.

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Risk Factors

• Maternal age >25 years

• Obesity (BMI ≥30 kg/m²)

• Previous macrosomic infant (>4.0 kg)

• History of GDM in prior pregnancy

• Family history of type 2 diabetes (first-degree relative)

• Polycystic ovary syndrome (PCOS)

• Multiple pregnancy (twins, triplets)

• Certain ethnic backgrounds (e.g., South Asian, Middle Eastern, African, Pacific Islander)

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Clinical Features

• Often asymptomatic – detected by screening

• Occasionally symptoms of hyperglycaemia:

  • Increased thirst

  • Polyuria

  • Fatigue

  • Recurrent urinary or vaginal infections

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Screening and Diagnosis

Screening Recommendations:

• Universal screening at 24–28 weeks gestation

• Early screening in high-risk women (at first antenatal visit)

Diagnostic Tests:

1. Two-step approach (common in the US):

   • Step 1: 50 g oral glucose challenge test (non-fasting); plasma glucose measured at 1 hour

     • If ≥7.8 mmol/L (140 mg/dL) → proceed to step 2

   • Step 2: 100 g oral glucose tolerance test (OGTT), fasting, with plasma glucose measured at 0, 1, 2, and 3 hours

     • Diagnosis if ≥2 values exceed thresholds:

       • Fasting ≥5.3 mmol/L (95 mg/dL)

       • 1 h ≥10.0 mmol/L (180 mg/dL)

       • 2 h ≥8.6 mmol/L (155 mg/dL)

       • 3 h ≥7.8 mmol/L (140 mg/dL)

2. One-step approach (WHO/IADPSG criteria):

   • 75 g OGTT, fasting, with glucose measured at 0, 1, and 2 hours

   • Diagnosis if any value meets or exceeds:

     • Fasting ≥5.1 mmol/L (92 mg/dL)

     • 1 h ≥10.0 mmol/L (180 mg/dL)

     • 2 h ≥8.5 mmol/L (153 mg/dL)

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Maternal and Fetal Complications

Maternal:

• Increased risk of preeclampsia

• Polyhydramnios

• Higher rate of operative delivery (caesarean section)

• Increased lifetime risk of type 2 diabetes

Fetal/Neonatal:

• Macrosomia (birth weight >4.0–4.5 kg)

• Birth trauma (shoulder dystocia, brachial plexus injury)

• Neonatal hypoglycaemia (due to hyperinsulinaemia)

• Respiratory distress syndrome

• Hyperbilirubinaemia

• Increased risk of obesity and metabolic syndrome later in life

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Management

Goals of Therapy:

• Maintain maternal euglycaemia

• Prevent fetal and maternal complications

• Avoid hypoglycaemia

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1. Lifestyle Modification

• Medical nutrition therapy: Individualized meal plan with controlled carbohydrate intake, high fiber, low glycaemic index foods

• Physical activity: Moderate-intensity exercise (e.g., walking 30 min/day) unless contraindicated

• Weight management: Avoid excessive gestational weight gain per guidelines

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2. Blood Glucose Monitoring

• Self-monitoring of blood glucose (SMBG) recommended:

  • Fasting and 1–2 h postprandial measurements

• Targets (typical):

  • Fasting: ≤5.3 mmol/L (95 mg/dL)

  • 1 h postprandial: ≤7.8 mmol/L (140 mg/dL)

  • 2 h postprandial: ≤6.7 mmol/L (120 mg/dL)

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3. Pharmacologic Therapy

Initiated if lifestyle measures fail to achieve glycaemic targets within 1–2 weeks.

First-line pharmacologic option:

• Insulin – does not cross placenta, safe in pregnancy

Common insulin regimens:

• NPH insulin for basal coverage: initial dose ~0.2 units/kg at bedtime

• Short-acting insulin (regular insulin, insulin lispro, or insulin aspart) before meals: starting dose often 4–6 units pre-meal, adjusted according to SMBG

• Total daily dose typically ranges from 0.7 units/kg (1st trimester) to 1.0 units/kg (3rd trimester), individualized

Oral hypoglycaemic agents (used in some settings when insulin is not feasible; both cross placenta to some extent):

• Metformin: Starting dose 500 mg orally once daily, titrated up to 2500 mg/day in divided doses

• Glibenclamide (glyburide): Starting dose 2.5–5 mg orally once daily, titrated to max 20 mg/day; less preferred due to higher risk of neonatal hypoglycaemia

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4. Antenatal Care and Delivery Planning

• Increased fetal surveillance (non-stress tests, growth scans)

• Delivery timing individualized; poorly controlled GDM may require induction at 38–39 weeks

• Caesarean section considered for estimated fetal weight ≥4.5 kg due to risk of shoulder dystocia

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Postpartum Care

• Most women return to normal glucose metabolism after delivery

• 6–12 weeks postpartum: 75 g OGTT to check for persistent diabetes or prediabetes

• Annual diabetes screening thereafter

• Lifestyle modification to reduce risk of type 2 diabetes

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Prognosis

• With appropriate management, outcomes approach those of non-GDM pregnancies

• Recurrence in subsequent pregnancies: ~30–70%

• Long-term maternal risk: ~50% develop type 2 diabetes within 10 years without intervention