Lucan · Conditions

Gestational diabetes mellitus

Australian guidance: KEMH (WNHS) GDM guideline v1.0 (30 Apr 2026) citing ADIPS June 2025 consensus

[!danger] DIAGNOSTIC THRESHOLDS WERE CORRECTED ON 2026-09-05 — read before revising from this file This note previously taught the superseded ADIPS 2014 / IADPSG thresholds of 5.1 / 10.0 / 8.5 mmol/L. The current figures, per the KEMH (WNHS) guideline v1.0 issued 30 April 2026 citing the ADIPS June 2025 consensus, are 5.3 / 10.6 / 9.0 mmol/L — all three RAISED. A result diagnostic under the old criteria may not be under the current ones.

What was corrected: the two prose threshold statements, the threshold cloze card, and SBA S1 (whose vignette carried a 1-hour value of 10.4 mmol/L keyed as diagnostic — true at 10.0, false at 10.6, so the question had no correct option; the value is now 10.8).

What was NOT re-verified — treat with suspicion: five other fact items in this file still carry GUIDELINE_VERSION: ADIPS 2014 (screening timing, treatment targets, and related content). Their claims were not checked against the 2026 KEMH guideline. Re-verify before relying on them.

1. Definition and epidemiology

Gestational diabetes mellitus (GDM) is carbohydrate intolerance resulting in hyperglycaemia that is first detected during pregnancy and is distinct from pre-existing type 1 or type 2 diabetes. In Australia it is the fastest growing type of diabetes: diagnoses more than doubled between 2008 and 2019, with almost 41,000 women diagnosed in 2019 alone, and over 500,000 Australian women are expected to develop GDM in the next decade. Risk is highest in women who are older, overweight or obese, of First Nations, South Asian, East Asian, Middle Eastern, African, or Melanesian/Polynesian backgrounds, or who have a personal or family history of GDM or type 2 diabetes.

2. Pathophysiology

Pregnancy induces a state of progressive insulin resistance driven by placental hormones, principally human placental lactogen, progesterone, oestrogen, cortisol, and prolactin. In most pregnancies, pancreatic beta-cell mass and secretory capacity expand to compensate. GDM develops when this compensatory response is insufficient, either because pre-existing beta-cell reserve is reduced or because placental counter-regulatory hormone output is unusually high. The resulting relative insulinopaenia leads to fasting hyperglycaemia, impaired postprandial glucose disposal, and increased hepatic glucose output. Foetal hyperinsulinaemia follows, because foetal beta-cells respond to glucose that crosses the placenta freely via facilitated diffusion (Robbins Ch 24). Insulin itself does not cross the placenta, so the foetus becomes its own source of the anabolic stimulus that drives macrosomia, organomegaly, and adiposity. Adipokine dysregulation and low-grade systemic inflammation further amplify maternal insulin resistance as pregnancy advances.

3. Clinical presentation and diagnostic criteria

Most women with GDM are asymptomatic; the diagnosis is made through scheduled screening rather than symptom-driven testing. Occasionally, women present with polyuria, polydipsia, or recurrent candidiasis, but these features are non-specific in pregnancy. On clinical assessment, risk factors to note include body mass index above 30 kg/m2, previous macrosomic infant (birthweight above 4,500 g), polycystic ovary syndrome, and current use of antipsychotic or glucocorticoid medications.

Diagnostic criteria in Australia follow the Australasian Diabetes in Pregnancy Society (ADIPS) consensus, updated in June 2025 and adopted by WA Health. GDM is diagnosed on a 75-gram oral glucose tolerance test (OGTT) performed between 24 and 28 weeks gestation when any one of the following venous plasma glucose values is met or exceeded: fasting 5.3 mmol/L, 1-hour post-load 10.6 mmol/L, or 2-hour post-load 9.0 mmol/L. A single abnormal value is sufficient.

4. Investigations and AU-specific cutoffs

The primary investigation is the 75-gram OGTT performed at 24 to 28 weeks gestation in all pregnant women not previously screened. Plasma glucose (not whole blood or point-of-care testing) must be used. Current ADIPS thresholds (June 2025 consensus, adopted by KEMH April 2026) are fasting 5.3 mmol/L, 1-hour 10.6 mmol/L, and 2-hour 9.0 mmol/L — raised from the 2014 figures of 5.1/10.0/8.5. They remain well below the WHO general-population thresholds (fasting 7.0, 2-hour 11.1), reflecting the additional foetal risk from more modest degrees of hyperglycaemia in pregnancy.

HbA1c is not recommended for diagnosing GDM because increased erythropoiesis in pregnancy lowers HbA1c relative to prevailing glucose levels and reduces diagnostic sensitivity. HbA1c may be useful at the first antenatal visit to detect previously undiagnosed overt diabetes (HbA1c at or above 6.5% alongside a confirmatory plasma glucose). Additional investigations on diagnosis include baseline HbA1c to characterise glycaemia before intervention, urine albumin-to-creatinine ratio if overt diabetes is suspected, blood pressure measurement, and foetal growth ultrasound.

5. Management

Management follows a stepped-care model anchored in the ADIPS guidelines and supported by national Diabetes Australia guidance.

Initial therapy is lifestyle modification: medical nutrition therapy delivered by an accredited practising dietitian, targeting a carbohydrate-controlled, low-glycaemic-index eating pattern, combined with moderate aerobic exercise such as 30 minutes of brisk walking on most days. Self-monitoring of blood glucose (SMBG) four times daily (fasting and 1 or 2 hours postprandial) guides assessment of glycaemic control. Target glucose levels are fasting below 5.0 mmol/L and 1-hour postprandial below 7.4 mmol/L (or 2-hour below 6.7 mmol/L).

Pharmacotherapy is commenced if targets are not met within one to two weeks of lifestyle intervention. Insulin is the preferred first-line pharmacological agent in Australia because it does not cross the placenta and has the most established safety record. Metformin is an evidence-based alternative that many women prefer due to oral administration, but it crosses the placenta and its long-term offspring safety data beyond childhood are still maturing. The ADIPS position accepts metformin as a reasonable option after informed consent when insulin is declined or where specific barriers exist, noting that up to half of women commencing metformin will require supplementary insulin to meet targets. The choice of insulin regimen depends on the pattern of glucose excursions: basal insulin only for isolated fasting hyperglycaemia, prandial rapid-acting insulin for postprandial excursions, or a combined basal-bolus regimen for mixed patterns. Dose titration is guided by SMBG results. All women with GDM should receive care from a multidisciplinary team that includes an obstetrician, credentialled diabetes educator, accredited practising dietitian, and, where appropriate, an endocrinologist or physician with expertise in diabetes in pregnancy.

6. Pharmacology and AU PBS notes

Insulin is available on the PBS for GDM and does not require a streamlined authority code when prescribed by a medical practitioner for diabetes in pregnancy. Commonly used insulins include:

Rapid-acting analogues for prandial use: insulin aspart (NovoRapid) and insulin lispro (Humalog), both PBS-listed. Dosing is individualised, typically commencing at 2 to 4 units with each meal and titrated against 1-hour postprandial targets.

Intermediate to long-acting insulins for basal use: Neutral Protamine Hagedorn (NPH, isophane) insulin (Protaphane, Humulin NPH) is the preferred basal insulin in pregnancy given the longest safety dataset; insulin detemir (Levemir) has also been studied in pregnancy and is an accepted alternative. Insulin glargine (Lantus) is used in practice but has less pregnancy-specific evidence than detemir.

Metformin (generic brands and Diabex, Glucophage) is not PBS-listed specifically for GDM but may be prescribed under the GDM indication. The typical starting dose is 500 mg once or twice daily with meals, titrated to a maximum of 2,500 mg daily in divided doses.

7. Complications and monitoring

Maternal complications include pre-eclampsia, caesarean delivery, postpartum haemorrhage, and perineal trauma from macrosomic infants. Women with GDM are seven times more likely than the general pregnant population to develop type 2 diabetes in later life, with approximately 60 per cent converting within 10 to 20 years of the index pregnancy. GDM recurs in up to 70 per cent of subsequent pregnancies.

Foetal and neonatal complications include macrosomia, shoulder dystocia, birth injury, neonatal hypoglycaemia (requiring blood glucose monitoring and early feeding protocols), respiratory distress syndrome, and hypocalcaemia. Babies born to mothers with GDM are more than twice as likely to require admission to a special care nursery or neonatal intensive care unit compared with babies of mothers without diabetes.

Monitoring schedule during pregnancy: SMBG four times daily throughout treatment; foetal growth ultrasound at 28 and 36 weeks; obstetric review fortnightly to assess growth, liquor volume, and blood pressure.

Postpartum monitoring: SMBG and insulin or metformin are ceased at delivery. A 75-gram OGTT is performed at 6 to 12 weeks postpartum to exclude persistent overt diabetes using standard non-pregnancy thresholds (fasting at or above 7.0 mmol/L or 2-hour at or above 11.1 mmol/L). If normal, fasting glucose or HbA1c should be checked every 1 to 2 years indefinitely, as the residual lifetime risk of type 2 diabetes remains substantial. Lifestyle modification and, where appropriate, metformin reduce the rate of progression.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.