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What is gestational diabetes?
Gestational diabetes is a form of diabetes that occurs during pregnancy. It is marked by high blood sugar (glucose) levels in pregnant women who ordinarily do not have diabetes. It commonly occurs between the 24th and 28th week of pregnancy, and in some cases even earlier. Gestational diabetes affects around 5% of pregnancies in Australia. 
This form of diabetes is caused by normal pregnancy hormones, which are released by the placenta. These hormones reduce the normal function of the mother's insulin. Insulin is an important chemical released by the pancreas to regulate blood sugar levels. In the presence of pregnancy hormones, normal levels of insulin are no longer able to control blood sugar levels, which is called insulin resistance. Normally, the mother's body produces up to two to three times more insulin than usual, to compensate for the insulin resistance. However, in gestational diabetes not enough insulin is produced, resulting in high blood sugar levels that may affect the pregnancy and the unborn baby's health.
The condition commonly settles soon after birth, as the pregnancy hormones disappear. However, it may increase the risk of developing diabetes in the future.
Glucose is an essential energy source for the body's cells. When food containing carbohydrates is eaten, it is broken down into glucose that travels around the body in the blood, to be absorbed by cells to use as an energy source. Insulin, a hormone produced by the pancreas, functions to help cells take in the glucose from the blood, thus regulating the blood glucose levels.
During pregnancy, hormones (such as growth factor, prolactin, progesterone and cortisol) are produced by the placenta to help the baby grow and develop. These same hormones can reduce the actions of the mother's insulin, leading to insulin resistance.
If insufficient insulin is produced to compensate for the insulin resistance, blood glucose levels will remain abnormally high, as seen in gestational diabetes. The consequence of high blood glucose levels during pregnancy can be:
To the unborn baby:
- A large baby, which increases the chance of complications during the delivery, such as difficulty passing through the birth canal;
- Low blood glucose levels in the baby soon after birth, as a result of overcompensation with relatively high insulin levels;
- Seizures, and;
To the mother:
- Increased risk of pre-eclampsia, which is a serious condition in pregnancy marked by high blood pressure, protein in the urine, visual disturbances, facial swelling and abdominal pain;
- Increased chance of requiring a caesarean section, as a result of difficult delivery due to a large baby, and;
- Increased risk of type 2 diabetes in the future.
Factors that can increase the risk of developing gestational diabetes include:
- Being more than 30 years of age;
- Having gestational diabetes in a previous pregnancy;
- Having polycystic ovary syndrome (PCOS);
- Having had a baby in a previous pregnancy whose weight at birth was more than 4.5 kilograms;
- Being overweight or obese;
- Having a family history of type 2 diabetes or gestational diabetes, and;
- Taking steroids or antipsychotic medications.
Women from some cultural and ethnic groups are also at a higher risk, including:
- Aboriginal Australians and Torres Strait Islanders;
- People of Indian descent;
- People of Vietnamese and Chinese descent;
- People of Middle Eastern descent, and;
- People of Polynesian or Melanesian background.
Signs and symptoms
Women with gestational diabetes often have no noticeable symptoms. However, they can include:
- Increased thirst;
- Increased hunger,
- Increased frequency of urination;
- Tiredness and fatigue;
- Unexpected weight loss;
- Infections of the bladder, skin and vagina;
- Nausea and vomiting, and;
- Blurred vision.
Methods for diagnosis
Gestational diabetes is assessed using two different laboratory tests, which both test for insulin resistance in the mother.
Glucose challenge test (GCT)
In this test, no fasting or change to your usual diet is required before the test is performed. You will have a blood test to measure the level of glucose in the blood. This will give a baseline-level blood glucose. You will then be given a drink containing glucose (either 50 or 75 grams of glucose) and an hour later another blood sample will be taken to measure the difference in your blood glucose level. Test results greater than 7.8 (for 50 grams of glucose) or 8.0 mmol/L (for 75 grams of glucose) indicate a need for further testing. This test does not diagnose gestational diabetes, but indicates if you are at increased risk and need further testing.
It is important to remember that many women do not experience any noticeable symptoms with gestational diabetes, therefore it is recommended that all pregnant women be screened by means of a glucose challenge test when they are between 26-28 weeks' gestation.
Oral glucose tolerance test (OGTT)
This test is performed if the glucose challenge test indicates possible gestational diabetes. Prior to the test, you will need to fast overnight. You will have a blood test to measure your blood glucose level in the morning. You will then be given a drink containing 75 grams of glucose and after one and two hours, further blood tests are taken.
Gestational diabetes is diagnosed if:
- The fasting test result is greater than 5.5 mmol/L, and/or;
- The result at one or two hours is 8.0 mmol/L or greater.
Types of treatment
If you are diagnosed with gestational diabetes, you will usually receive additional care from doctors, specialists and other healthcare professionals during your pregnancy. The key to treatment of gestational diabetes is controlling blood glucose levels. This can be achieved using different treatments:
Most women with gestational diabetes can control their condition using lifestyle measures such as eating a healthy diet and being physically active.
Eating a well-balanced diet can help with maintaining healthy blood glucose levels. A diet that is low in fat, with plenty of fruits, vegetables and whole grains is recommended. If you need help planning your meals, a dietitian can provide guidance.
Regular physical activity is an important part of controlling blood glucose. However, it is important to talk to your doctor about what activities are suitable and any precautions you may need to take.
Maintaining a healthy weight can help you to manage gestational diabetes and stay healthy during pregnancy. As every woman's situation is different, your doctor can advise you on what weight goals may be suitable for you.
Monitoring your own blood glucose levels at home is important to assess if the treatments are working. This is done using a blood glucose machine, which analyses your glucose level from a blood sample obtained by a finger prick. You will be taught how to perform self-testing by your doctor or a diabetic nurse.
The frequency of testing will depend on the fluctuation of your blood glucose levels and the type of treatment you receive, which your doctor can advise you on.
Some women with gestational diabetes require treatment with insulin to keep their blood glucose levels under control. Insulin needs to be injected and cannot be taken as a tablet.
There are many different types of insulin that have different periods of activity including:
- Rapid-acting insulin, which starts working in 5-15 minutes and peaks 30-90 minutes after injecting;
- Short-acting insulin, which starts working 30-60 minutes after injection and generally peaks in 2-4 hours;
- Intermediate-acting insulin, which starts working 1-3 hours after it is taken and peaks in eight hours, and;
- Long-acting insulin, which can provide coverage for as long as 20-26 hours.
Effects on the baby
Babies of mothers who have gestational diabetes tend to be larger and weigh more at birth. This increases the risk of birth injury.
They may experience more episodes of low blood glucose levels (hypoglycaemia) in the first few days.
If gestational diabetes is not treated, there is a slightly increased risk of the baby dying.
Effects on the mother
Due to a larger baby and the increased risk of complications with delivery, the mother has an increased likelihood of needing a caesarean section.
There is an increased risk of low blood glucose levels (hypoglycaemia). It can happen to people taking medication for diabetes, particularly if they delay or miss a meal, drink alcohol or are more physically active than usual.
If blood sugar levels fall too low, symptoms can include:
- Feeling weak, light-headed or dizzy;
- Trembling and shaking;
- Feeling hungry;
- Difficulty concentrating;
- Feeling irritable or teary, and;
- Numbness around the fingers or lips.
Severe hypoglycaemia is a serious condition and can cause people to lose consciousness. Monitoring blood glucose levels and taking early steps to bring them back to normal can help to prevent hypoglycaemia.
Increased risk of type 2 diabetes
While blood glucose tends to return to normal levels after birth, women who have had gestational diabetes have a greater risk of developing type 2 diabetes later in life, often within 5-10 years.
Women who have had gestational diabetes are recommended to have a glucose tolerance test 6-12 weeks after giving birth and then a fasting blood glucose test every three years after that. A fasting blood glucose test is a blood sample taken after an overnight fast.
With appropriate lifestyle management and treatment, most women with gestational diabetes can manage blood glucose levels well and avoid serious health consequences for themselves and their babies.
Gestational diabetes cannot always be prevented, but taking measures before pregnancy to achieve a healthy weight can help to reduce your risk.
Prenatal care can help to diagnose gestational diabetes early and identify any other health problems that may occur during pregnancy.