What is gestational diabetes mellitus?

Gestational diabetes mellitusDiabetes is a condition in which blood sugar (glucose) levels are high. This is due to problems with the hormone insulin, which allows sugar (mainly digested from the diet) to be taken up from the blood into the cells and to be processed as the body’s main energy source. If insulin levels are too low, or the body does not respond normally to insulin, sugar is ‘trapped’ in the blood (known as ‘glucose intolerance’), and diabetes occurs.

Gestational diabetes mellitus (GDM) is glucose intolerance that is first diagnosed during pregnancy (gestation).

GDM is a temporary condition that usually disappears after pregnancy. However, for some women the condition becomes chronic. Rarely, diabetes may have been present before pregnancy. If it has not been diagnosed previously, it is still known as GDM. Even if it goes away, GDM signals an increased risk of developing diabetes later in life, which can have serious short term and long term effects on the body. It is important that GDM is diagnosed and treated, because without treatment it may lead to significant health problems for a mother and her baby.

Statistics of gestational diabetes mellitus

About 5% of pregnant women in Australia develop GDM. This means that around 11,000 cases are diagnosed each year in Australia. The rates of GDM are increasing. This may be linked to the increasing average age of women having children.

Rates of GDM are 2.3 times higher in Indigenous Australian women than non Indigenous Australian women. Other women with high risk ethnic backgrounds are those from the Pacific Islands, Asia, the Middle East, or the Indian subcontinent.

Risk factors for gestational diabetes mellitus

Predisposing factors for GDM include:

  • Age (risk increases with age). As women age, their body’s ability to compensate for the hormonal changes of pregnancy is reduced.
  • Obesity or being overweight. People who are overweight are likely to have a greater baseline level of glucose intolerance.
  • Family history of diabetes. There is a genetic link to risk of diabetes.
  • Ethnic background (People from the Pacific Islands, Asia, the Middle East and the Indian subcontinent, and Indigenous Australians are at higher risk).
  • History of glucose intolerance or GDM. This indicates that the body is predisposed to problems with glucose tolerance.
  • History of delivering ‘large for gestational age’ babies. This indicates that there may have been problems with blood sugar levels in previous pregnancies.
  • Polycystic ovarian syndrome
  • High blood pressure during pregnancy

The Australasian Diabetes in Pregnancy Society (ADIPS) recommends screening for all pregnant women, because GDM may occur in women with no identifiable risk factors.

A regular program of exercise before pregnancy appears to lower the risk of developing GDM.

Progression of gestational diabetes mellitus

The levels of hormones that lead to GDM increase as the pregnancy progresses, particularly during the second and third trimester. This means that blood sugar control is more difficult later in the pregnancy. If blood sugar levels increase in the mother and baby, the baby will become overweight, and the amount of fluid in the amniotic sac will increase. This may make the process of delivering the baby more difficult.

GDM usually disappears after pregnancy, though the mother is at increased risk of developing permanent diabetes later in life. It is therefore important that she continues to have a healthy diet and exercise to minimise this risk.

Symptoms of gestational diabetes mellitus

Gestational diabetes mellitusIf a pregnant mother’s blood sugar levels remain too high, or uncontrolled, there is a risk that the baby will become overweight. Large babies may make the birthing process more difficult. The baby is also more likely to be overweight as a child, which increases the child’s risk of health problems in the future, such as high blood pressure, heart disease and diabetes. These risks are reduced significantly by learning to keep blood sugar levels within the normal range.

Clinical examination of gestational diabetes mellitus

Clinical examination of women with GDM may be normal. Monitoring weight gain, blood pressure, and blood sugar of the pregnant woman is essential.

It is also essential to monitor the developing baby for complications.

How is gestational diabetes mellitus diagnosed?

All pregnant women should have a screening test for GDM, because treatment can significantly reduce the risks of harm to the mother and baby associated with the condition.

Screening is carried out at 26-28 weeks into pregnancy, though it should be earlier if the mother is felt to be at high risk of undiagnosed diabetes. GDM is more likely to be diagnosed in the last 12-14 weeks of pregnancy as hormone levels are higher.

The screening test is done in the morning, and involves drinking a set amount of glucose. A blood test is done to measure the levels of glucose in the blood an hour later. If this level is high, the test is repeated on another morning, with the mother required to fast (not eat) for 10-12 hours beforehand.

Women who are at high risk of GDM have the ‘fasting’ test in the first instance (they skip the screening test). This will be done early in the pregnancy. If it is normal, it is tested again at 28 weeks.

Prognosis of gestational diabetes mellitus

If blood sugar is not well controlled during pregnancy, the following outcomes have been associated with GDM:

  • The baby being born overweight (> 4 kg)
  • Increased requirement for forceps delivery, episiotomy or caesarean section (due to the large size of the baby)
  • Increased risk of death of the baby around the time of delivery
  • Preeclampsia (high blood pressure during pregnancy, along with reduced kidney function)
  • Increased amount of amniotic fluid in the sac surrounding the baby in the uterus
  • Problems with the infant’s ability to manage substances such as calcium, bile, and sugar.

Potential long term consequences to the infant of a mother with GDM include:

High blood sugar at the time GDM is diagnosed, despite fasting, has been associated with increased risks of birth defects in the newborn. This is more common in mothers with pre-existing diabetes than GDM.

Treatment of gestational diabetes mellitus

Gestational diabetes mellitusBlood glucose levels need to be checked daily (using a finger prick test) as recommended by a Diabetes Educator or doctor.

The main treatment of GDM is diet therapy, aimed at controlling blood glucose levels. A healthy balanced diet, with very little saturated fat and processed carbohydrates (sugars) is required. If blood glucose control is not adequate despite appropriate gentle exercise and diet, insulin injections may be necessary. Between 10 and 25% of all women with gestational diabetes need to use insulin injections as part of their treatment. Insulin should not have a negative effect on the baby, as it does not cross into the baby’s blood supply.

Minimum goals of therapy (determined by blood tests) are:

  • Fasting blood glucose < 5.5 mmol/L
  • 1 hour post meal < 8.0 mmol/L
  • or 2 hours post meal < 7.0 mmol/L

It is essential to closely monitor the baby throughout the pregnancy for complications. All being well, pregnancy may be continued in uncomplicated GDM for up to 10 days past the due date.

After the baby is born, an early blood test (via a heel prick) will be done to check that the blood sugar levels have not dropped too low. Early breastfeeding (within half an hour of birth) will help to control the infant’s blood sugar levels.

Because GDM indicates a women is at higher risk of developing permanent diabetes, ADIPS recommends that mothers with GDM be tested 6-12 weeks following birth of the baby, and again every 2-3 years.


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