Gestational Diabetes

Clinical implications of GDM

Women with GDM have an increased risk of obstetric and neonatal complications, including pre-eclampsia, preterm delivery, obstetric intervention, macrosomia, neonatal hypoglycaemia, respiratory distress syndrome and jaundice.^2,3 Exposure to hyperglycaemia in utero also results in an increased metabolic risk in the longer term. Detection of GDM is important, as treatment reduces the risk of obstetric and perinatal complications.

Incidence

Gestational diabetes prevalence is increasing. According to a study including more than 1.8 million women, the incidence of GDM in Australia has increased significantly in recent years, from 8.9% in 2016 to 14.8% in 2021.¹ Higher rates are seen with increasing maternal age, lower socioeconomic status and in certain ethnic groups. Women from South and Central Asia had the highest incidence, followed closely by women born in Southeast Asia.

Possible reasons for the marked increase in GDM observed in Australia include increasing maternal age, increasing prevalence of overweight or obesity in pregnant women and migration resulting in a higher proportion of pregnant women from high-risk ethnic groups.

Risk factors for GDM

Table 1. Risk factors for gestational diabetes from the Australian Diabetes in Pregnancy 2025 Guidelines³

Diagnosis

The Australian Diabetes in Pregnancy Society (ADIPS) has recently updated the recommendations for diagnosis of gestational diabetes. Due to a review of recent evidence, the recommended glucose thresholds for diagnosis of gestational diabetes have been increased.

The earlier 2014 ADIPS cut-offs for diagnosis of GDM were based on data from the large international Hyperglycaemia and Adverse Pregnancy Outcomes (HAPO) study. This study showed increased risk for perinatal complications associated with increasing maternal glucose concentration following a 75g oral glucose tolerance test (OGTT). While the risk was continuous, glucose thresholds corresponding to an odds ratio of 1.75 (compared to the mean glucose) were adopted. Due to recent data showing a potential for overtreatment at the lower thresholds, as well as increased awareness of the potential psychosocial impact of a diagnosis of GDM, the recommended thresholds have been increased to correspond with an odds ratio of 2.0 from the HAPO study.

The updated recommendations have been widely endorsed or accepted across concerned organisations within Australia, including: the Australian College of Midwives (ACM), the Australian Diabetes Society (ADS), Diabetes Australia (DA), the Endocrine Society of Australia (ESA), the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG), the Royal College of Pathologists of Australasia (RCPA), the Royal Australian College of General Practitioners (RACGP) and the Society of Obstetric Medicine of Australia and New Zealand (SOMANZ). 

Diagnostic criteria (ADIPS 2025)³

Gestational diabetes diagnosis is based on any one of the following during a 75g 2-hour OGTT (performed at any time during pregnancy):

• Fasting plasma glucose ≥ 5.3–6.9 mmol/L
• 1-hour plasma glucose ≥ 10.6 mmol/L
• 2-hour plasma glucose ≥ 9.0–11.0 mmol/L

Overt diabetes in pregnancy should be diagnosed at any gestation if one or more of the following criteria are met:

• Fasting plasma glucose ≥ 7.0 mmol/L or 2-hour plasma glucose ≥ 11.1 mmol/L following a 75g 2-hour OGTT
• HbA1c ≥ 6.5% (≥ 48 mmol/mol)
• Random plasma glucose ≥ 11.1 mmol/L in the presence of clinical signs or symptoms indicative of hyperglycaemia

Recommended approach to screening

Early testing for women with risk factors

• HbA1c at the first antenatal visit
  Women with one of more risk factors for hyperglycaemia in pregnancy (Table 1) should have an HbA1c performed at the first antenatal visit, if not already performed within the past 12 months, to identify pre-existing, but undiagnosed overt diabetes (HbA1c ≥ 6.5%).
• Early OGTT
  Women with a previous history of gestational diabetes mellitus or early pregnancy HbA1c levels 6.0–6.4% (without a history of diabetes) should have a 75g 2-hour OGTT before 20 weeks’ gestation (ideally between 10 and 14 weeks). While not universally recommended, an OGTT in early pregnancy may also be offered to women with other risk factors based on the women’s informed decision or local policies.

Due to limited tolerance related to nausea and uncertain benefit, OGTT should not be performed before 10 weeks’ gestation.

Universal testing at 24-28 weeks’ gestation

All women (without diabetes already detected in the current pregnancy) should have a 75g 2-hour OGTT at 24–28 weeks’ gestation.

Suggested approach when an OGTT is not performed

An OGTT may not be performed in some women, either due to it not being tolerated or due to individual choice. An OGTT is not recommended in women with a history of bariatric surgery due to potentially inaccurate results and reactive hypoglycaemia related to altered gastric emptying.

For women where an OGTT is not performed, a fasting plasma glucose is recommended, and a result ≥ 5.3 mmol/L should be managed as gestational diabetes. Women with a fasting glucose < 5.3 mmol/L in early pregnancy can wait until further screening at 24-28 weeks’ gestation but should be advised that GDM cannot be excluded without an OGTT.

For women with fasting plasma glucose 4.7-5.3 mmol/L at 24-28 weeks’ gestation, a period of capillary self-blood glucose monitoring may be considered, although there is no guidance on glucose thresholds for GDM diagnosis based on self-blood glucose monitoring, and government subsidies for glucose monitoring equipment are not available without diagnosed diabetes.

Due to a physiological fall in HbA1c by the second trimester, HbA1c is not recommended to screen for gestational diabetes in later pregnancy due to poor sensitivity. However, women with an HbA1c 6.0-6.4% who have not had an OGTT should be offered self-blood glucose monitoring and dietary education.

Postpartum follow-up

Women who are diagnosed with gestational diabetes should have an OGTT at 6-12 weeks postpartum to assess maternal glucose status.

Due to the increased risk of type 2 diabetes and cardiometabolic disease, regular screening for diabetes and assessment of cardiovascular risk factors is recommended.

Transitioning to the 2025 ADIPS criteria

There will be some women who have been diagnosed with gestational diabetes based on the previous ADIPS 2014 criteria. For these women, ADIPS recommends continuing care based on the original diagnosis.⁴ Decisions regarding blood glucose monitoring, treatments, or the model or care provided should consider current self-monitored blood glucose levels and the overall clinical context.

Women diagnosed based on the 2014 ADIPS criteria still have an increased risk of developing type 2 diabetes, for which screening and diabetes prevention advice is recommended.

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