Obstetric Topic 12. Pregnancy and diabetes mellitus. Gestational diabetes

I. Classification and Pathophysiology

Classification

• Pregestational diabetes: type 1 or type 2 DM present before conception.
• Gestational diabetes mellitus (GDM): glucose intolerance with onset/first recognition during pregnancy, usually in the 2nd-3rd trimester.
• Early severe hyperglycemia may represent previously undiagnosed type 2 DM rather than true GDM.

Risk Factors for GDM

• Higher maternal age (>30 years).
• Obesity, especially BMI >35.
• Family history of DM.
• Previous GDM.
• Previous macrosomic infant, malformed infant or unexplained perinatal loss.
• Associated conditions: corticosteroid use, PCOS, dyslipidemia, hypertension/metabolic syndrome.
• Glycosuria or impaired fasting glucose/glucose tolerance before pregnancy.

Pathophysiology

• Placental hormones, especially hPL/hCS + progesterone, cortisol and prolactin → ↑ insulin resistance.
• Insufficient maternal beta-cell compensation → maternal hyperglycemia.
• Glucose crosses placenta; maternal insulin does not cross placenta.
• Maternal hyperglycemia → fetal hyperglycemia → fetal hyperinsulinemia → macrosomia, organomegaly, neonatal hypoglycemia.
• Poor pregestational control during organogenesis → teratogenic hyperglycemia → miscarriage and congenital malformations.
• Diabetic vasculopathy → placental insufficiency → IUGR risk.

II. Complications

Maternal Complications

• Polyhydramnios.
• Preeclampsia and chronic hypertension complications.
• Infections: UTI, pyelonephritis, vulvovaginal candidiasis.
• Preterm labor and difficult labor due to macrosomia → shoulder dystocia, operative delivery, cesarean.
• Diabetic ketoacidosis, hypoglycemia in insulin-treated patients.
• Worsening diabetic complications: retinopathy, nephropathy, neuropathy and cardiovascular disease.
• After GDM: ↑ risk of recurrent GDM and future type 2 DM.

Fetal and Neonatal Complications

• Macrosomia: fetal weight >4000 g → shoulder dystocia and birth trauma.
• IUGR: mainly with maternal vascular disease/placental insufficiency.
• Stillbirth/IUFD, especially with poor control, DKA or vascular disease.
• Congenital malformations in pregestational DM: neural tube, cardiac, skeletal defects, caudal regression.
• Delayed organ maturity → respiratory distress syndrome.
• Neonatal hypoglycemia, hypocalcemia, hypomagnesemia, polycythemia and hyperbilirubinemia.

III. Screening and Diagnosis

When to Screen

• Universal GDM screening: GW24-28.
• Earlier screening if high-risk or symptoms/signs suggest overt diabetes.
• If early test is negative → repeat routine GDM screening at GW24-28.
• Do not perform OGTT in known pregestational DM; monitor glucose and HbA1c instead.

One-Step Approach: 75 g OGTT

• Fasting 75 g OGTT with fasting, 1 h and 2 h plasma glucose; any one abnormal value diagnoses GDM.
• Fasting: ≥92 mg/dL (5.1 mmol/L).
• 1 h: ≥180 mg/dL (10.0 mmol/L).
• 2 h: ≥153 mg/dL (8.5 mmol/L).

Two-Step Approach

• 1st step: 50 g glucose challenge test, non-fasting, 1 h.
• Screen positive: commonly ≥140 mg/dL (7.8 mmol/L), local cutoff may be 130-140 mg/dL.
• 2nd step: fasting 100 g OGTT; GDM if ≥2 values are above threshold.
• Overt DM suggested by fasting ≥126 mg/dL (7.0 mmol/L) or random/2 h ≥200 mg/dL (11.1 mmol/L).

IV. Management During Pregnancy

Preconception Care for Pregestational Diabetes

• Optimize glycemic control before organogenesis; HbA1c target ideally <6.5% if safely achievable.
• Stop teratogenic drugs before pregnancy when possible: ACE inhibitors/ARBs and statins.
• Folic acid supplementation.
• Screen diabetic complications: retinopathy, nephropathy/albuminuria, BP and cardiovascular disease if indicated.
• Type 1 DM: continue insulin; educate about hypoglycemia and DKA.

Diet, Exercise and Monitoring

• First-line for GDM: diet + physical activity + glucose monitoring.
• Diet: regular meals, controlled carbohydrates, avoid sugary drinks, include protein/fiber, avoid starvation/ketosis.
• Exercise: mild-moderate activity if no obstetric contraindication.
• Monitor blood glucose: fasting and postprandial values; HbA1c mainly for pregestational DM.
• Targets: fasting <95 mg/dL, 1 h postprandial <140 mg/dL, or 2 h postprandial <120 mg/dL.

Insulin and Fetal Surveillance

• Insulin if diet/exercise fails or glucose is repeatedly above target; preferred drug when pharmacologic treatment is needed.
• Oral agents are used in some systems, but insulin is often preferred because metformin/glyburide cross the placenta.
• US: congenital abnormalities in pregestational DM, fetal growth/macrosomia/IUGR and amniotic fluid.
• Fetal echocardiography: consider in pregestational DM due to congenital cardiac defect risk.
• NST/CTG or BPP: often from GW32 in pregestational, insulin-treated, poorly controlled or complicated diabetes.

V. Delivery, Postpartum and Emergencies

Timing and Mode of Delivery

• Well-controlled diet-treated GDM: await spontaneous labor at term; avoid post-term pregnancy.
• Well-controlled insulin-treated GDM: induction/planned delivery around GW38-39 according to local protocol.
• Poorly controlled or complicated diabetes: earlier delivery may be needed after maternal-fetal risk assessment.
• Suspected diabetic macrosomia: consider planned C-section when estimated fetal weight ≥4500 g.
• Intrapartum: frequent maternal glucose checks, maintain euglycemia, IV insulin/dextrose if needed.

Postpartum Care

• Placenta delivered → insulin-antagonist hormones fall → insulin requirement drops sharply.
• GDM: usually resolves after delivery; stop insulin/oral therapy unless hyperglycemia persists.
• Pregestational DM: reduce insulin dose postpartum and monitor for hypoglycemia.
• Postpartum test after GDM: 75 g OGTT at 4-12 weeks postpartum (6-12 weeks in many local notes).
• Long-term follow-up: screen for type 2 DM every 1-3 years; counsel contraception and next pregnancy planning.

Diabetic Emergencies

• Hypoglycemia: sweating, tremor, confusion, unconsciousness → oral glucose or glucagon/IV glucose if severe.
• DKA: nausea/vomiting, abdominal pain, dehydration, tachypnea, ketones, acidosis → obstetric-medical emergency.
• Treatment of DKA: fluids + insulin + potassium/electrolyte correction + treat trigger; stabilize mother first.