Gestational diabetes: screening, glucose targets, treatment, and postnatal care

Gestational diabetes mellitus is the most common medical complication of pregnancy in Australia. It affects approximately 15% of pregnancies and the rate is rising, driven by increasing prevalence of obesity, older maternal age at first pregnancy, and the adoption of more sensitive diagnostic criteria since 2014.

The condition matters not only for the pregnancy itself but for what it reveals about long-term metabolic risk — for the mother, who faces a lifetime type 2 diabetes risk approaching 50%, and for the baby, whose intrauterine environment shapes metabolic health into adulthood.

A. Core clinical — the AU general-practice framework

What gestational diabetes is

GDM is hyperglycaemia of any degree first recognised during pregnancy. The pathophysiology involves pregnancy-induced insulin resistance — driven by placental hormones — combined with insufficient insulin secretory reserve to compensate. This typically becomes apparent after 20 weeks when placental hormone production peaks.

Three categories of glucose abnormality in pregnancy are important to distinguish:

• GDM — hyperglycaemia meeting ADIPS thresholds below, first detected in pregnancy, not meeting criteria for overt diabetes
• Diabetes in pregnancy (DIP) — overt diabetes detected in pregnancy (fasting ≥7.0 mmol/L, 2-h OGTT ≥11.1 mmol/L, or HbA1c ≥6.5%); managed as pre-existing diabetes
• Pre-existing T1/T2 DM — ideally identified preconception with specific preconception protocols

Screening — who, when, how

ADIPS and RANZCOG recommend universal screening at 24–28 weeks for all pregnant women using a 75 g two-hour OGTT. This replaced the older risk-factor-only approach — Australian data show risk-factor-based screening misses approximately 50% of GDM.

High-risk women should be screened earlier at 12–16 weeks:

• BMI ≥30
• Previous GDM
• First-degree family history of type 2 diabetes
• High-risk ethnicity: Aboriginal or Torres Strait Islander, South Asian, Pacific Islander, Middle Eastern, African, or East Asian
• Age ≥40
• Prior macrosomic baby (≥4.5 kg)
• PCOS
• Glycosuria at booking visit
• Long-term corticosteroid or antipsychotic use

If early screening is normal, repeat at 24–28 weeks.

OGTT preparation: overnight fast 8–12 hours (water permitted); diet containing at least 150 g carbohydrate daily for three days beforehand; plasma glucose at fasting, one hour, and two hours after a 75 g glucose load.

Diagnostic thresholds (IADPSG/ADIPS 2014; WHO 2013)

GDM is diagnosed when any one of the following is met:

• Fasting ≥5.1 mmol/L
• 1-hour ≥10.0 mmol/L
• 2-hour ≥8.5 mmol/L

Management framework

Initial management follows a stepped approach under eTG and ADIPS guidance:

1. Education — referral to a diabetes educator for glucose monitoring technique, interpretation, and target understanding
2. Medical nutrition therapy (MNT) — accredited practising dietitian referral; low-GI carbohydrates distributed across 5–6 smaller meals; adequate protein; limiting refined carbohydrates and sugar-sweetened beverages
3. Physical activity — 30 minutes of moderate aerobic exercise on most days (walking, swimming, cycling); resistance exercise is also appropriate in uncomplicated pregnancy
4. Blood glucose monitoring (BGM) — four-times daily (fasting + post-each-main-meal); NDSS registration provides subsidised test strips and lancets
5. Pharmacotherapy if targets not met within approximately two weeks

Glucose targets (ADIPS)

• Fasting: less than 5.0–5.3 mmol/L
• 1-hour post-meal: less than 7.4 mmol/L
• 2-hour post-meal: less than 6.7 mmol/L

B. Evidence basis — what the landmark trials established

Treatment of GDM improves outcomes

The ACHOIS trial (Crowther et al., NEJM 2005) randomised 1,000 women with mild GDM to routine care versus active treatment. Active treatment reduced serious perinatal complications (including macrosomia, shoulder dystocia, and Caesarean section) without increasing maternal harm. The MFMU trial (Landon et al., NEJM 2009) confirmed similar findings in a further 958 women with mild GDM. Together, these trials established that treating even mild GDM changes outcomes — making the threshold for diagnosis and treatment clinically important.

The diagnostic threshold — HAPO and IADPSG criteria

The HAPO study (NEJM 2008) demonstrated a continuous, graded relationship between maternal blood glucose and adverse outcomes (macrosomia, neonatal hypoglycaemia, Caesarean section, pre-eclampsia) with no clear threshold. This underpinned the 2010 IADPSG diagnostic criteria — now adopted by ADIPS — which use lower glucose thresholds than older criteria and therefore diagnose more women.

Metformin versus insulin

The MiG trial (Rowan et al., NEJM 2008) randomised 751 women with GDM to metformin or insulin. Metformin was non-inferior for primary outcomes including neonatal hypoglycaemia and composite perinatal complications, with less maternal weight gain. Approximately 46% of women randomised to metformin required supplemental insulin, so starting metformin does not always avoid insulin. Metformin crosses the placenta; its long-term offspring effects are the subject of ongoing research but current evidence does not identify a safety signal.

C. Antenatal monitoring, delivery, and postnatal care

Antenatal surveillance

Women with GDM require multidisciplinary input and serial monitoring:

• Growth ultrasound at 28–32–36 weeks to detect macrosomia or growth restriction
• Fetal wellbeing assessment (CTG, biophysical profile) in the third trimester, especially if control is suboptimal or macrosomia present
• Blood pressure and urine protein/creatinine ratio monitoring — GDM increases risk of pre-eclampsia
• Regular review by the GDM clinic (obstetrician, diabetes educator, dietitian) — typically public hospital antenatal or private obstetric service

Pharmacotherapy — initiation

When MNT and activity do not achieve targets, the options per eTG and ADIPS are:

Metformin (Category C): start 500 mg daily with food, titrate to 500–2,000 mg/day in divided doses. GI side effects are common; dose-escalation over weeks reduces them. Not first-line if DIP (overt diabetes in pregnancy) is present.

Insulin: gold-standard, particularly for fasting hyperglycaemia and when metformin is insufficient. Rapid-acting analogues (aspart/NovoRapid, lispro/Humalog — both Category A) cover post-meal glucose; long-acting (detemir/Levemir — Category A; NPH/protaphane) address fasting glucose. Doses are initiated by the GDM clinic or endocrinologist and titrated to target. All listed insulins are PBS-available under Authority Required (Streamlined) criteria.

Glibenclamide (glyburide) is not recommended as first-line — comparative trials have shown less reassuring neonatal outcomes relative to metformin and insulin.

Delivery timing

• Diet-controlled GDM with good glucose control — typically 39–40 weeks
• GDM on pharmacotherapy, or with macrosomia or poor control — typically 37–39 weeks
• Estimated fetal weight ≥4.5 kg at term — discussion regarding elective Caesarean section to reduce shoulder dystocia risk

Postnatal care — the frequently missed step

GDM medication is stopped at delivery in most cases — glucose typically normalises rapidly as the placenta is delivered.

Breastfeeding is strongly encouraged. It reduces the mother’s subsequent type 2 diabetes risk by 30–50% and reduces the infant’s long-term risk of obesity and diabetes.

75 g OGTT at 6–12 weeks postpartum is an ADIPS strong recommendation. Approximately 5–15% of women will be found to have undiagnosed type 2 diabetes, and a further proportion will have impaired fasting glucose or impaired glucose tolerance. This test is frequently not ordered — a care gap worth actively closing.

Long-term screening: OGTT or fasting glucose every 1–3 years ongoing. GDM also predicts higher lifetime cardiovascular disease risk independent of subsequent T2DM.

D. Australian operations

MBS items

• GP consultations: items 23, 36, 44
• Antenatal shared care attendances: items 16590, 16591
• High-risk antenatal review: item 16622
• OGTT (pathology bundle): item 73807
• Diabetes educator under EPC plan: item 10951 (up to 5 allied health visits/year under GPCCMP)
• Accredited practising dietitian: item 10954
• Exercise physiologist: item 10968
• GPCCMP preparation/review: items 965/967 (applicable postpartum for women with persisting glucose abnormality)
• ATSI Health Assessment: item 715

PBS

• Metformin IR and XR — general schedule
• Insulin (all listed basal and bolus preparations) — Authority Required (Streamlined)
• Insulin pump consumables — specific program access (mainly T1DM)

NDSS — National Diabetes Services Scheme

Women with GDM can register with the NDSS for subsidised blood glucose test strips, lancets, and syringes. Registration is completed by the GP or midwife. The NDSS also provides education resources in multiple languages and a 24-hour helpline.

E. Special populations

Aboriginal and Torres Strait Islander women. Prevalence of GDM is approximately 20% in ATSI populations. Cultural and dietary tailoring of MNT advice is essential. Interpreter services and culturally appropriate diabetes educator resources are available via NDSS. ATSI Health Assessment (item 715) supports proactive identification.

PCOS. Women with polycystic ovary syndrome have significantly elevated GDM risk — screen early (12–16 weeks) and offer pre-conception lifestyle support and weight management to reduce risk.

Obesity (BMI ≥30). The most modifiable risk factor for GDM. Preconception weight loss reduces GDM risk. During pregnancy, dietary counselling should aim for appropriate — not maximal — gestational weight gain per Institute of Medicine guidelines.

Multifetal pregnancies. Higher GDM risk; higher glucose targets may apply in some clinical contexts — specialist obstetric guidance recommended.

Women needing insulin in pregnancy. This does not indicate poor self-management — it indicates the degree of insulin resistance exceeds what diet and oral agents can address. Insulin initiation by a GDM clinic with specialist nurse educator support reduces barriers.

When to escalate

Refer to or discuss with specialist GDM services when:

• Glucose targets are consistently not met with MNT alone
• Insulin initiation or titration is required
• Suspected overt diabetes in pregnancy (DIP — fasting ≥7.0, 2-h ≥11.1, HbA1c ≥6.5%)
• Fetal macrosomia identified on growth ultrasound
• Pre-eclampsia or other obstetric complications develop
• Very early GDM diagnosis (first trimester) — suggests possible pre-existing undiagnosed T2DM

All women with GDM at a public hospital will typically be referred to a specialised antenatal GDM clinic. Private obstetric models vary — the GP’s role in ensuring referral and postnatal follow-up is particularly important.

What this article is and is not

This is general health information based on ADIPS and RANZCOG guidelines, NHMRC antenatal care guidance, Therapeutic Guidelines, and the landmark ACHOIS, MFMU, MiG, and HAPO trials. It does not constitute personal medical advice. GDM management is individualised — glucose targets, medication choices, and delivery planning require input from the treating obstetric and diabetes team who know the full clinical picture.

For consumer-level information: NDSS — Gestational diabetes, HealthDirect — Gestational diabetes, Diabetes Australia, and Better Health Channel.

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Sources cited

1. ADIPS — Australasian Diabetes in Pregnancy Society Consensus Guidelines
2. RANZCOG — Gestational diabetes mellitus
3. NHMRC — Clinical Practice Guidelines for Pregnancy Care
4. Therapeutic Guidelines (eTG)
5. Diabetes Australia
6. National Diabetes Services Scheme (NDSS)
7. HealthDirect — Gestational diabetes
8. Better Health Channel — Gestational diabetes
9. Crowther CA et al. — ACHOIS trial (NEJM 2005)
10. Landon MB et al. — MFMU trial (NEJM 2009)
11. Rowan JA et al. — MiG trial (NEJM 2008)
12. HAPO Study Cooperative Research Group (NEJM 2008)