19 July 2021

Study raises doubt about new GDM criteria

Clinical Diabetes Women

Lowering the cut-off for diagnosing gestational diabetes hasn’t appeared to reduce harms, and may have instead increased the rate medical interventions.

A study of NSW perinatal and hospital data found that the incidence of gestational diabetes more than doubled between 2006 and 2015, with a sharp rise in 2011, when the new thresholds were introduced.

Associate Professor Sarah Glastras, endocrinologist at Sydney’s Royal North Shore Hospital, and colleagues found that widening the criteria to include more women with gestational diabetes had resulted in a greater number of interventions, such as planned births and caesarean sections.

In addition, the controversial guideline change did not appear to have improved perinatal outcomes as hoped.

Fewer babies were born with macrosomia to women with gestational diabetes, but more babies were large for gestational age.

Professor Glastras attributed the reduction in fetal macrosomia to women having their delivery induced earlier, as there were more planned births before the 39th week.

“Despite increased intervention, there were no clear improvements in the composite maternal morbidity indicator, and in fact, maternal morbidity increased slightly over the study period from 1.4% (2006) to 1.9% (2015),” Professor Glastras wrote.

“Neonatal morbidity also increased, and there was a doubling in the rate of neonatal hypoglycemia in the total cohort from 2.4% to 4.8%.”

Perinatal mortality did drop, although this was also true for women without gestational diabetes – suggesting diagnostic change wasn’t responsible, she said.

Professor Glastras said inducing births earlier would have implications for the mother and child, such as on establishing feeding, and it was still unclear whether the benefits outweighed the risks.

“But certainly, from a health system point of view, it is a huge cost to have to induce women earlier, potentially increasing the rates of caesarean section,” she said.

Moreover, once women are labelled as having gestational diabetes, they may face a much more stressful pregnancy.

“You may have been previously seeing a midwife or a GP for your care, then suddenly that has to stop and then you get moved into a tertiary hospital system to have your antenatal care,” said Professor Glastras.

They may have to travel much greater distances for their antenatal care, incur travel and parking expenses, risk more interactions during a pandemic, change their diet and lifestyle, and check their blood sugar levels multiple times per day.

“At the end of the day, you wonder why this needs to happen; if in fact these women had a mild condition and don’t need that sort of high level of care, or that high-risk pregnancy model,” she said.

Around one in six Australian women who become pregnant are diagnosed with gestational diabetes, and their children are at risk of worse short- and long-term outcomes.

The decision to lower the threshold for gestational diabetes has been hotly debated, with medical groups such as RANZCOG and the WHO informed by research that maternal glucose levels were linked to adverse outcomes on a spectrum rather than a strict cut-off.

The move has also been criticised for medicalising women who wouldn’t previously have been labelled with a condition, and risking overdiagnosis and overtreatment.

The increase in incidence of gestational diabetes cannot be entirely explained by the change in diagnostic thresholds. Some of the dramatic increase in GDM rates over the past two decades could be attributed to the population becoming heavier, older and increasingly from at-risk ethnicities.

Nonetheless Professor Glastras and colleagues pin much of the uptick on the new criteria.

Ideally, this change would have led to an improvement in perinatal outcomes, particularly for the babies, said Professor Glastras.

“But in fact, we don’t see that they changed at all,” she said. “So, it leads us to conclude that it hasn’t really done much other than diagnose a whole lot of more women with gestational diabetes, which, of course, carries a huge burden.”

A better approach would be one tailored to the individual’s risk, including factors such as age, family history and body weight, she said.

Professor Glastras said that the findings were likely to be similar across the country, although health districts here and globally have varied in implementing the criteria because of the resulting expense for the health system.

BMJ Open Diabetes Research & Care 2021, online 28 June

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