Description
- Two distinct entities managed differently
- Gestational diabetes (GDM) - carbohydrate intolerance first recognised in pregnancy
- Pre-existing (pregestational) DM - T1DM or T2DM predating conception; pregnancy superimposed on established disease
- Overt diabetes in pregnancy - meets non-pregnant diabetes criteria at booking -> managed as pre-existing, not GDM
Epidemiology
- GDM ~15-20% of Australian pregnancies (rising with new lower diagnostic thresholds and obesity prevalence)
- Pre-existing DM ~1% of pregnancies
- Risk factors: prior GDM, BMI >=30, FHx T2DM, PCOS, prior macrosomia (>4.5 kg), South/East Asian, Aboriginal and Torres Strait Islander, Pacific, Middle Eastern ethnicity, age >40, prior unexplained stillbirth
Aetiopathogenesis
- Pregnancy = physiological insulin resistance
- Placental hormones - hPL, progesterone, cortisol, prolactin - antagonise insulin, greatest effect 2nd/3rd trimester
- GDM when beta-cell reserve cannot compensate
- T1DM - absolute insulin deficiency
- inc hypoglycaemia risk 1st trimester (nausea, unpredictable intake, insulin sensitivity transiently improves)
- inc requirement from ~16-20 weeks as resistance rises
- Pedersen hypothesis - maternal hyperglycaemia -> fetal hyperinsulinaemia -> fetal fat/organ overgrowth, delayed lung maturation
- Periconceptional hyperglycaemia (pre-existing DM only) -> teratogenic - cardiac defects, neural tube defects, sacral agenesis - risk proportional to HbA1c at conception
Diagnosis
Screening (ADIPS 2025)
- HbA1c at booking if risk factors present
- >=6.5% = overt diabetes, treat as pre-existing
- 6.0-6.4% = repeat 75g OGTT before 20 weeks
- All women: 75g 2h OGTT at 24-28 weeks
ADIPS 2025 diagnostic thresholds - one value diagnostic
| Timepoint | Threshold (mmol/L) |
|---|---|
| Fasting | >=5.3 |
| 1 hour | >=10.6 |
| 2 hour | >=9.0 |
- Thresholds raised from the 2014 IADPSG-derived criteria - fewer women now labelled GDM at the margin
Pre-existing DM
- Known diagnosis before conception, or
- Booking FPG >=7.0 mmol/L or HbA1c >=6.5% -> overt/pregestational diabetes
Management
Preconception (known pre-existing DM)
- HbA1c target <6.5-7% before conception
- High-dose folate 5 mg/day from preconception
- Stop teratogens: ACEi/ARB, statins - switch to pregnancy-safe agents
- Baseline retinal exam + renal function; repeat each trimester (retinopathy can progress rapidly)
Glycaemic targets (all types)
- Fasting <5.0-5.3 mmol/L
- 1h post-prandial <7.4-7.8 mmol/L
- 2h post-prandial <6.7-7.0 mmol/L
By severity
- GDM, diet-controlled (~70-80%) - MNT + exercise, self-monitoring 4x/day
- GDM/T2DM not controlled by lifestyle - insulin first-line
- Metformin - reasonable alternative/adjunct; crosses placenta; less maternal hypoglycaemia and weight gain, higher secondary insulin requirement; long-term offspring metabolic effects still debated
- Glibenclamide - less favoured, higher rates of macrosomia and neonatal hypoglycaemia than insulin
- T1DM - continuous insulin (pump or basal-bolus), rising requirement through pregnancy; hypoglycaemia awareness education
Fetal surveillance
- Growth ultrasound every 4 weeks from 28-32 weeks - macrosomia, polyhydramnios
- Fetal echocardiography if pre-existing DM with poor early control
Timing and mode of birth
- Diet-controlled GDM, normal growth - can await spontaneous labour to 40+6
- Insulin-treated GDM or T2DM - offer birth 38-39 weeks
- T1DM or vasculopathy - individualise, often earlier
- Estimated fetal weight >4.5 kg - counsel on caesarean to reduce shoulder dystocia risk
Intrapartum
- Target BGL 4-7 mmol/L in labour (sliding-scale insulin/dextrose infusion if pre-existing DM or poorly controlled GDM)
- Minimises neonatal hypoglycaemia
Postpartum
- GDM - stop insulin/metformin immediately, 75g OGTT at 6-12 weeks postpartum
- Pre-existing DM - revert to pre-pregnancy regimen (insulin requirement falls sharply after placental delivery)
- Breastfeeding encouraged - reduces future maternal and offspring T2DM risk
- Contraception counselling before discharge
Associations
- Pre-eclampsia (~2x risk), polyhydramnios
- Macrosomia -> shoulder dystocia, birth trauma, operative delivery
- Congenital malformation risk (pre-existing DM only) - cardiac defects, neural tube defects, caudal regression - correlates with periconceptional HbA1c
- Neonatal: hypoglycaemia, respiratory distress (surfactant maturation delayed), hypocalcaemia, polycythaemia, hyperbilirubinaemia, hypertrophic cardiomyopathy
- Stillbirth risk inc with poor control, especially pre-existing DM
Natural history & complications
- GDM glucose tolerance normalises postpartum in most
- Future T2DM risk ~7x background - up to 50% within 10 years
- Recurrence in subsequent pregnancy ~40-60%
- Pre-existing DM - pregnancy can accelerate retinopathy and nephropathy; usually regresses postpartum but needs review
- Offspring: inc lifelong obesity and T2DM risk (fetal programming)
7 of 7 sections written · drafted 2026-09-13