Gestational Diabetes Diagnosis – Why Does Where I Live Matter?

Gestational diabetes is a condition that can seriously impact the health and well-being of pregnant women and babies. However, variations in when and how testing is conducted mean that where a woman lives can significantly impact her diagnosis.

Additionally, in Australia, a change in how a diagnosis is made has seen a three-fold increase in the number of women diagnosed with gestational diabetes. This diagnosis carries consequences that can negatively impact birth choices and general quality of life.

Who decides who should be tested for gestational diabetes?

Who we test for gestational diabetes is a widely debated topic between countries. For example, in the UK, the National Institute for Health and Care Excellence (NICE) is responsible for publishing evidence-based guidelines to inform clinical practice nationally, including who should be tested for gestational diabetes.

Based on these guidelines, only women considered at risk of developing gestational diabetes are tested in the UK. The risk factors considered are:

• a BMI greater than 30 kg/m²
• a previous large baby of 4.5+ kg
• previous gestational diabetes
• a family history of diabetes
• ethnicity with a high prevalence of diabetes

In addition, the NICE guidelines recommend that women who have had gestational diabetes in a previous pregnancy be offered early self-monitoring of blood glucose and/or an early OGTT. Australia, however, does not follow the NICE guidelines and instead recommends all women be tested for gestational diabetes at 24 to 28 weeks.

How does Australia test for gestational diabetes?

Australia uses the oral glucose tolerance test (OGTT) to diagnose gestational diabetes. The test requires fasting (not eating) for 8 to 10 hours and then drinking a measure of glucose (usually 75g) after a baseline blood test. Additional blood tests are then taken at intervals (usually 1 hour and 2 hours) to measure your blood glucose (sugar) levels and how quickly and effectively your body is processing glucose.

An alternative testing method, the Carpenter–Coustan two-step criteria, involves women first drinking a 50g glucose challenge test and then taking a blood sample at 1 hour to determine blood glucose levels. The threshold for this test varies between 7.2 and 7.8 mmol/L. Women may then undergo an OGTT test with blood tests at 1 hour, 2 hours and 3 hours.

However, this is where it gets tricky. The one-step OGTT has been shown to result in more women being classified as having gestational diabetes than the two-step Carpenter–Coustan criteria. In addition, how many blood test results are considered in either test will alter the results, which raises the obvious question – if different countries use different tests and testing procedures, who is right and what are the potential impacts?

Different countries mean a different diagnosis

It is accepted that different countries have various standards of medical practice. Governments base decisions on a range of factors. Current legislation, availability of resources, economic wealth, the standard of the public health system and recent research all affect medical practice. However, what makes this an even more complicated debate is that there is no consistent evidence of the level at which blood glucose levels start to become dangerous for women and babies.

In fact, in 2008, the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) Study Cooperative Research Group published an article based on research conducted on 25,505 pregnant women across nine countries who underwent testing for gestational diabetes. The results of the HAPO report state ‘there were no obvious thresholds at which risks increased’.

We do not have definitive research-based evidence to show at what level a woman’s blood glucose can be considered a problem in pregnancy. What we do have is a range of contradictory international research. The result is a variance between countries in the testing methods and thresholds used to determine gestational diabetes. The table below demonstrates this.

You might notice that, of the three countries represented, Australia currently has the lowest levels at which blood glucose is considered a problem. Also, since 2010, when Australia moved from a 2-step process to a 1-step process, fasting levels have dropped lower, and measurement at 1-hour after fasting has been added.

These changes are vital because, since 2010, Australia has seen the percentage of women diagnosed with gestational diabetes triple from around 5% in 2010 to 16% in 2018. This change has not occurred because we suddenly saw more women with elevated blood sugar levels but because we lowered the bar on testing.

What impact does a change in the testing threshold have on Australian women?

Australia changed its testing regime for gestational diabetes in response to a position statement published in 2008 by the International Association of Diabetes and Pregnancy Study Groups (IADPSG). The IADPSG was formed to review the findings of the HAPO Study noted above.

However, because the IADPSG recommendations were not evidence-based, they were rejected by many countries and widely criticised for adversely impacting women’s quality of life during pregnancy, birth and their babies.

However, following the recommendations places Australian women at a greater risk for adverse impacts from a gestational diabetes diagnosis than women in countries employing evidence-based testing procedures. The Royal Australian College of General Practitioners, in particular, has highlighted several negative impacts on women previously considered healthy but now being classified as high-risk.

What to read more?

The decision by Australian medical authorities to follow the IAPSG recommendations was highly controversial. It raises serious concerns regarding the impact of testing methods for gestational diabetes in Australia on women and babies.

To read more about the effect on women and babies of Australia’s decision to follow the IADPSG recommendations or to explore the symptoms, testing procedures, risks and complications of gestational diabetes, consider reading the following articles:

Published 12th July 2022