Gestational Diabetes: A Guide for Australians

Gestational diabetes mellitus (GDM) is a form of diabetes that develops during pregnancy in women who did not have diabetes before becoming pregnant. It is one of the most common complications of pregnancy in Australia, affecting approximately 1 in 7 pregnancies — a rate that has been increasing over time, partly due to rising rates of overweight and obesity in the population and changes in diagnostic criteria.

A GDM diagnosis can feel alarming, but with appropriate management it is a condition that most women navigate successfully, with good outcomes for both mother and baby. This guide explains what gestational diabetes is, how it is diagnosed in Australia, how it is managed, and what to expect after your baby is born.

What Causes Gestational Diabetes?

During pregnancy, the placenta produces hormones that help support the growing baby. Some of these hormones — including human placental lactogen, progesterone and cortisol — have the effect of reducing the mother's sensitivity to insulin. This is normal and happens in all pregnancies to some degree, ensuring the baby has a sufficient supply of glucose.

In most pregnancies, the pancreas compensates by producing more insulin. In gestational diabetes, the pancreas cannot keep up with the increased demand, and blood glucose levels rise above normal. This typically begins in the second trimester, when placental hormone production is at its highest.

Gestational diabetes is not caused by eating too much sugar, though diet influences blood glucose levels once GDM is diagnosed.

Risk Factors

Any pregnant woman can develop gestational diabetes, but the following factors increase the risk:

• Being overweight or obese before pregnancy
• Family history of type 2 diabetes in a first-degree relative
• Having had gestational diabetes in a previous pregnancy
• Being aged 35 or over
• Being from certain ethnic backgrounds, including South Asian, East Asian, Middle Eastern, Indigenous Australian, Māori and Pacific Islander backgrounds
• Having polycystic ovarian syndrome (PCOS)
• Having previously given birth to a large baby (over 4.5 kg)
• Multiple pregnancy (twins or more)

How Gestational Diabetes Is Diagnosed in Australia

In Australia, screening for gestational diabetes is recommended for all pregnant women between 24 and 28 weeks of pregnancy using an oral glucose tolerance test (OGTT). Women at high risk may be tested earlier, sometimes in the first trimester.

The OGTT involves:

1. Fasting overnight (for at least 8 hours)
2. A fasting blood glucose test
3. Drinking a 75g glucose solution
4. Blood glucose tests at 1 hour and 2 hours after the glucose drink

Gestational diabetes is diagnosed if any one of the following thresholds is met or exceeded, as per the Australasian Diabetes in Pregnancy Society (ADIPS) guidelines:

Blood Glucose Targets During Pregnancy

Blood glucose targets during pregnancy are considerably tighter than the general targets for adults with diabetes outside of pregnancy. This is because elevated blood glucose in pregnancy is associated with risks to both mother and baby, including macrosomia (larger-than-normal baby), neonatal hypoglycaemia, preterm birth and increased likelihood of caesarean delivery.

These targets are managed in close collaboration with your obstetric team and a diabetes specialist or endocrinologist. Do not attempt to manage gestational diabetes targets without the direct guidance of your healthcare team.

Managing Gestational Diabetes

Diet and lifestyle

The majority of women with gestational diabetes manage their blood glucose through dietary changes and appropriate physical activity, without needing medication. Working with an Accredited Practising Dietitian experienced in gestational diabetes is strongly recommended. General principles include:

• Spreading carbohydrate intake evenly across three main meals and two to three snacks per day, rather than consuming large amounts at one sitting
• Choosing lower glycaemic index carbohydrates, which cause a slower and more moderate glucose rise
• Including protein and healthy fat with each meal to moderate glucose absorption
• Avoiding sugary drinks, including fruit juice
• Regular gentle physical activity such as walking — check with your obstetrician about what is appropriate for your stage of pregnancy

Blood glucose monitoring

Most women with gestational diabetes are asked to monitor blood glucose at home using a fingerprick meter — typically fasting and one or two hours after each main meal. This provides the data your healthcare team needs to assess whether your blood glucose is within target and whether treatment needs to be adjusted.

Some women with gestational diabetes are now using CGM (continuous glucose monitoring) in pregnancy, though this is not yet standard practice across all Australian maternity services. Discuss with your team whether CGM is appropriate for your situation.

Medication

If diet and lifestyle changes are insufficient to maintain blood glucose within target, medication may be needed. The main options used in Australian practice are:

• Insulin — the most commonly used medication for gestational diabetes that requires pharmacological treatment; safe for use in pregnancy and does not cross the placenta
• Metformin — sometimes used as an alternative or addition to insulin; your specialist will discuss the evidence and considerations with you

After Your Baby Is Born

For most women, blood glucose levels return to normal shortly after delivery, once the placental hormones that caused insulin resistance are no longer present. Your blood glucose should be checked before you leave hospital and again at your follow-up appointment.

However, having gestational diabetes does increase the risk of developing type 2 diabetes later in life. Australian guidelines recommend:

• An OGTT at 6–12 weeks after birth to confirm blood glucose has returned to normal
• Ongoing monitoring with a fasting glucose or HbA1c test every 1–3 years thereafter
• Lifestyle measures — maintaining a healthy weight, staying physically active and eating well — to reduce the long-term risk of type 2 diabetes

Future Pregnancies

Women who have had gestational diabetes in one pregnancy have a significantly higher risk of developing it again in subsequent pregnancies. This risk is highest when weight gained between pregnancies is not lost, so maintaining a healthy weight before your next pregnancy is one of the most effective steps you can take to reduce the likelihood of GDM recurring.

Inform your GP and obstetric team about your history of gestational diabetes at the start of any future pregnancy, as earlier screening and closer monitoring may be recommended.