The Australasian Diabetes in Pregnancy Society (ADIPS) estimates 5%-8% of pregnancies in Australia are affected by gestational diabetes mellitus, and rates continue to rise.

Risk factors and screening guidelines

Recognised risk factors for gestational diabetes mellitus (GDM) include maternal age (>30 years), increasing BMI (>30 kg/m2), polycystic ovarian syndrome, ethnicity (e.g. Australian Indigenous, Polynesian, South-East Asian, Southern Asian), family history of type 2 diabetes mellitus (T2DM), and previous GDM or macrosomic baby.

Many women, however, do not have identifiable risk factors, and universal screening at 26-28 weeks' gestation is recommended by ADIPS (Table 1). Those considered to be at very high risk for GDM should be offered early (first trimester) screening. This includes women with a history of GDM.

The importance of identifying gdm

Untreated GDM is associated with foetal macrosomia, birth trauma, caesarean delivery, neonatal hypoglycaemia, hyperbilirubinemia and polycythemia.

The benefits of treating women with GDM were clearly demonstrated by the Australian Carbohydrate Intolerance Study in Pregnant Women (ACHOIS). A thousand women with GDM were randomised to routine antenatal care or an intensive regimen of self-glucose monitoring and lifestyle intervention +/- insulin.

Serious adverse perinatal outcomes occurred in 1% of the intervention group and 4% in the routine group, equating to a three-fold increase in risk to the baby. Large-for-gestational-age infants were less common (13% vs 22%) with intensive therapy. Induction of labour was increased (39% vs 29%) but caesarean section rates were similar (~31 per cent). Thirty-four women needed to be treated to prevent one serious perinatal outcome.

Improved maternal quality of life was also noted in the intensive group. Notably, based on traditional risk factors, many of these women would not have been screened for GDM.

The precise level of hyperglycaemia that imparts risk to the mother and foetus is unclear. The Hyperglycemia and Adverse Pregnancy Outcome Study (HAPO) of more than 23,000 women demonstrated an independent effect of glucose on pregnancy outcomes with a continuum of risk as a function of glucose levels, albeit without a clear threshold value. For example, macrosomia risk increased four- to six-fold from the lowest to the highest blood glucose value. It is not immediately obvious where ‘abnormal' begins, and analysis is ongoing. The definition of GDM is likely to be reviewed as a result of the HAPO study.

Management

Where possible, management should be in a multidisciplinary setting with an obstetrician, diabetes physician, diabetes educator, dietitian and midwife. Women are taught to self-monitor blood glucose levels.

Lifestyle intervention is the cornerstone of management. Regular meals and snacks with controlled portion sizes and low glycaemic index foods will decrease post-prandial glucose levels. Daily physical exercise for at least 30 minutes is advised if there are no contraindications. If blood sugar levels remain elevated insulin therapy is commenced; the type and dose frequency are based on individual requirements. The short-acting insulin analogues are now approved for use in pregnancy although the newer, longer acting analogues are not.

The role of oral hypoglycaemic agents is currently under review. The recently published Metformin in Gestational Diabetes Trial (MiG), a non-inferiority open label study of 750 women treated with insulin or metformin (+/- insulin), found no differences in neonatal outcomes and comparable birth weights. Almost half the women treated with metformin also required insulin.

Glyburide (glibenclamide), a sulphonylurea that does not cross the placenta, has also been studied in GDM and shown to provide similar metabolic control to insulin. There are no direct comparisons of the two oral hypoglycaemics.

Postpartum care

A postnatal oral glucose tolerance test and medical review at 6-8 weeks post delivery are indicated to identify persisting IGT/T2DM, to plan for future pregnancies, and to formulate a long-term diabetes screening and prevention strategy.

Contraception choices that are effective and convenient should be discussed. Limited data support the use of low-dose combined oral contraceptive pills. However, if other cardiovascular risk factors are present, consideration should be given to potentially metabolically neutral methods (e.g. IUD).

Women should be made aware that their offspring are at increased risk of obesity and abnormal glucose metabolism, both in childhood and adulthood.

Follow-up

Women with GDM have an increased lifetime risk of developing T2DM, the magnitude of which is dependent upon the presence of additional predictors of T2DM.

Up to 50%-70% of women from ethnic groups with high prevalence rates of T2DM are at risk. Notably, in women with IGT, the rate of progression to diabetes is greater in those with previous GDM when compared to those with normo-glycaemia in pregnancy. Between 10%-31% of parous women with T2DM have had GDM.

Education focused on exercise targets, healthy weight goals, regular screening and accurate risk perception is critical.

Lifestyle modification can prevent or delay the onset of T2DM. Recently, published data from the Diabetes Prevention Program found that either intensive lifestyle intervention or metformin reduced the incidence of T2DM by approximately 50% after three years - for every six women treated, one case of diabetes was prevented.

Conclusion

GDM rates in Australia are likely to continue to increase as antenatal populations become older with increasing BMIs and include higher-risk ethnic groups. Treatment paradigms may evolve to include oral medications in addition to insulin, but lifestyle intervention underpins management both during pregnancy and postpartum.              

The Jean Hailes Foundation for Women's Health is a national, non-profit health organisation focusing on practical education opportunities for health professionals and women.

Table 1. ADIPS guidelines for screening and diagnosis of GDM

[Source: MJA 1998;169:93-97]

  Gestational diabetes mellitus (139.98 KB)

References and further reading available at www.medicalobserver.com.au

Content Updated February 9, 2009