ObstetricsTier 2Topic

Risk factors for common pregnancy-associated diseases - diabetes

Definition and mechanism

Gestational diabetes mellitus (GDM) - glucose intolerance first recognised in pregnancy (distinct from pre-existing/overt diabetes unmasked by pregnancy)
Driven by placental hormones (human placental lactogen, progesterone, cortisol) progressively increasing insulin resistance through gestation, unmasking limited beta-cell reserve

Epidemiology

~12-15% of pregnancies in Australia (rising with newer, lower diagnostic thresholds and increasing maternal obesity/age)
Peak diagnosis 24-28 weeks (routine screening); earlier if high risk

Risk factors and early testing

Risk factors: BMI >=25 (>=30 higher risk), age >=40, family history of T2DM, previous GDM, PCOS, previous macrosomic infant (>=4.5kg), high-risk ethnicity (South Asian, Aboriginal/Torres Strait Islander, Pacific Islander, Middle Eastern), corticosteroid use, previous unexplained stillbirth
High-risk women -> early OGTT in the first trimester, repeated at 24-28 weeks if normal

Diagnosis - 75 g OGTT

75g 2-hour OGTT - diagnosis needs only ONE value at or above threshold (ADIPS 2025):

Timepoint	Threshold
Fasting	>=5.3 mmol/L
1 hour	>=10.6 mmol/L
2 hour	>=9.0 mmol/L

Overt diabetes in pregnancy (pre-existing diabetes unmasked): fasting >=7.0 mmol/L, 2h >=11.1 mmol/L, or HbA1c >=6.5% (48mmol/mol), at any gestation
These 2025 ADIPS thresholds replace the 2014 criteria - the fasting threshold is raised and a 1-hour value has been added

Management

Diet and exercise first-line, with self-monitored blood glucose
Metformin and/or insulin if targets not met
Insulin preferred if metformin is insufficient or contraindicated; metformin crosses the placenta but is not teratogenic
Timing of birth individualised - not usually before 38-39 weeks if well-controlled

Complications

Macrosomia, shoulder dystocia, neonatal hypoglycaemia (from fetal hyperinsulinaemia), polyhydramnios
Long-term: ~50% risk of progression to T2DM within 10 years

After delivery

Usually resolves after delivery, but marks a lifelong increase in T2DM risk
Postpartum 75g OGTT at 6-12 weeks is mandatory, followed by periodic screening thereafter

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