Gestational diabetes

Gestational diabetes (GDM; also referred to as gestational diabetes mellitus) is defined as glucose intolerance that begins, or is first diagnosed, during pregnancy. It may appear at any time pregnancy, particularly in women at high risk of GDM. 

Early gestational diabetes (eGDM) is defined as glucose intolerance (but not meeting the criteria for other forms of diabetes) that was diagnosed before 20 weeks gestation. 

Diabetes in pregnancy is defined by the World Health Organization as pregnant women whose blood glucose levels in pregnancy meet the criteria used to diagnose diabetes outside pregnancy.⁶ Some of these women may have previously undiagnosed diabetes (usually type 2). 

Most published data that report on GDM include both diabetes in pregnancy and GDM, and indeed most women will fit the specific criteria for GDM. 

Note that not all women with diabetes in pregnancy will continue to have diabetes following delivery. One Australian study reported that 41% of women with diabetes in pregnancy returned to normal glucose tolerance by 6–8 weeks postpartum.⁷ 

The 2008 Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study reported a correlation between increasing maternal glucose levels at 24–32 weeks gestation and a range of adverse maternal and fetal outcomes.⁸ The study suggested that the relationship between increasing blood glucose levels and adverse effects was continuous, with no threshold or inflection point at which lower levels confer protection. 

In response to the HAPO study, the International Association of the Diabetes and Pregnancy Study Groups developed new consensus guidelines for the testing and diagnosis of GDM.⁹ Although the Royal Australian and New Zealand College of Obstetricians and Gynaecologists and ADIPS³ have recommended that these consensus guidelines be implemented, there has been controversy nationally and internationally. Important differences in these guidelines are the universal testing of all women (not already diagnosed with diabetes) in pregnancy, and a one-step diagnostic framework with a changed glucose threshold (versus the previous two-step process). 

The early testing and diagnosis of GDM has substantial costs, both for women and the health system. Early screening for people with a high risk of pregnancy-associated diabetes (see Box1) may detect undiagnosed diabetes requiring intervention and support for healthier pregnancy outcomes. However, general earlier screening (<24–28 weeks gestation) for GDM has not been supported by clinical evidence except in selected cases (prior GDM or elevated glycated haemoglobin [HBA1c] at the initial screening visit [eg 6.0–6.4%]).¹⁰ The EGGO trial looked at early screening for gestational diabetes in obese women and did not show benefit.¹¹ However, the Treatment of Booking Gestational Diabetes Mellitus (ToBOGM) study went further and demonstrated that the immediate treatment of women diagnosed with GDM before 20 weeks (eGDM) led to a modest reduction in adverse neonatal outcomes (especially neonatal respiratory distress and days in the neonatal intensive care unit) and a reduction in adverse maternal outcomes (especially severe perineal tears); however, no material differences were observed for pregnancy-related hypertension or neonatal lean body mass.¹² 

It is important that each general practitioner (GP) is aware of their local obstetric service’s diagnostic criteria, and supports and manages patients in a manner congruent with their specialist team guidelines to avoid conflict and patient confusion. 

Identifying GDM 

Identifying women at risk of GDM, or who have previously undetected hyperglycaemia, enables the GP to advise women appropriately on risk minimisation and provide support and treatment. Hyperglycaemia is increasing in pregnancy parallel to rising rates of diabetes and obesity. Of women giving birth in 2015–16, approximately 15% were diagnosed with GDM.¹³ 

Australian clinical guidelines for care during pregnancy recommend that women who are at risk of hyperglycaemia, including GDM (Box 1), are tested in the first trimester of pregnancy (10–14 weeks gestation and up to 20 weeks gestation).² Women tested in the first trimester of pregnancy who have a normal test result should be advised to retest between 24 and 28 weeks gestation.² 

All pregnant women (without diabetes) should be advised to undergo testing for hyperglycaemia between 24 and 28 weeks gestation.² 

Discussion to inform a woman’s decision making about testing for hyperglycaemia should take place before testing. 

Diagnosing GDM 

Diagnostic criteria for GDM are presented in Box 2. HbA1c is not recommended to test for GDM due to a lack of sensitivity.² 

At present there are limited data demonstrating clinical benefit for women identified by the changed screening criteria compared with those identified by the 1997 ADIPS consensus criteria, which then became the National Health and Medical Research Council recommendations.^16–19 

Acknowledging that in Australian general practice there are alternative diagnostic criteria for GDM, the RACGP (preferred) and ADIPS (alternative) diagnostic criteria are both presented in Box 2. These remain the preferred criteria of the RACGP until evidence of such benefit is forthcoming, including the health economic costs of any such consensus for change. Furthermore, it is important that each GP is aware of their local obstetric service diagnostic criteria, and supports and manages patients in a manner confluent with their specialist team guidelines to avoid conflict and patient confusion. 

OGTT in pregnancy 

The correct procedure for OGTT is described in Box 2. 

Some women may vomit during the OGTT. In such cases, if the recorded fasting glucose meets the criteria for GDM, the woman should be referred to start GDM management. If a woman’s fasting glucose level is normal, repeat the OGTT with the woman taking metoclopramide beforehand. Metoclopramide does not appear to alter glucose absorption, but ondansetron may lead to falsely lower post-load glucose levels. Recliner chairs can also reduce the tendency to vomit. 

Women who have had metabolic surgery should not be sent for an OGTT because they may not be able to tolerate the test.²¹ Seek specialist advice from your local diabetes-in-pregnancy service regarding alternative testing options. 

Although none will equate to an OGTT, alternatives include giving a different source of 75 g carbohydrate, measuring blood glucose concentrations using continuous glucose monitoring, measuring fasting and postprandial blood glucose concentrations with capillary (fingerprick) blood testing, measuring HbA1c or using a combination of these methods. 

Women who have had metabolic surgery also need particular assessment throughout pregnancy regarding nutritional status, the need for higher multivitamin doses and close obstetric monitoring.²¹ Referral to appropriate specialty services is strongly advised prior to and during pregnancy, even for women in this group who do not have diabetes or GDM. 

Management of GDM 

Lifestyle interventions and insulin remain the mainstay of treatment for GDM. All women with GDM should be offered individualised management, including education, appropriate blood glucose monitoring and dietary advice. 

Education 

In most cases, GDM responds positively to lifestyle management, and women should be referred to an accredited practising dietitian and a credentialled diabetes educator, if these are not provided by their obstetric service. 

All women with GDM who qualify for Medicare access should be registered with the National Diabetes Services Scheme (NDSS) on the National Gestational Diabetes Register. 

If GDM diabetes is diagnosed during pregnancy, points for discussion include:² 

• the role of diet, physical activity and pregnancy/gestational weight gain in managing diabetes 
• that healthy dietary patterns can be supported by individualised advice from an accredited practising dietitian as part of the multidisciplinary team and, when appropriate, may be characterised by the intake of whole foods such as fruits, vegetables, legumes, wholegrains, fish, seafood, unprocessed meats, dairy foods and water 
• the role of insulin or metformin in the management of diabetes (ie if diet and physical activity do not adequately manage blood glucose levels) 
• the importance of monitoring and managing blood glucose levels during pregnancy, labour and birth and early feeding of the baby to reduce the likelihood of the baby having macrosomia and associated risks (eg fractures, shoulder dystocia, jaundice) 
• the possibility of the baby requiring admission to a special care nursery/neonatal intensive care unit to manage possible hypoglycaemia or respiratory distress 
• the woman’s increased future risk of developing type 2 diabetes and cardiovascular disease, and the importance of reviewing glucose tolerance postpartum and maintaining a healthy weight 
• the benefits of registering on the NDSS, including the National Gestational Diabetes Register (eg reminders for glucose tolerance assessment) 
• the benefits of breastfeeding in reducing the risk of the woman developing type 2 diabetes in the future 
• the risk of the baby developing obesity, heart disease and/or diabetes in the future. 

Follow-up of patients with a history of GDM 

Women diagnosed with GDM have an approximate 40% risk of recurrence of GDM in a subsequent pregnancy and an increased risk of developing future type 2 diabetes.²² Regular ongoing surveillance is required.³ Box 3 provides RACGP criteria for the follow-up of patients with a history of GDM. 

A review of medications needs to be assessed with any ongoing breast feeding and appropriateness of ongoing prescribing. Primary care provides an ideal environment to support the mother and infant in the postnatal period, including psychosocial and mental health support, lifestyle advice and support and, where appropriate, referral to maintain the health of the mother and child.