Gestational diabetes is defined as any degree of glucose intolerance with onset or first recognition during pregnancy. In pregnancy, the placenta produces hormones to support and sustain the developing baby. However, some of these hormones, such as human placental lactogen, can cause insulin resistance in the pregnant mother, resulting in a higher need for insulin whilst the mother is pregnant. If the body is unable to cope with the extra demand for insulin production, then this results in gestational diabetes. Most patients who have gestational diabetes return to normal glucose tolerance parameters postpartum and can still have a healthy baby. However, gestational diabetes needs to be managed in order to reduce the risk of developing complications during pregnancy, such as macrosomia of the baby, which means the fetus grows excessively large.

Risk factors for gestational diabetes include:

  • Gestational diabetes in a previous pregnancy
  • Previously elevated blood glucose concentrations prior to pregnancy
  • History of abnormal glucose tolerance
  • Age over 40 years
  • Ethnicity (Aboriginal and Torres Strait Islander, Maori and Pacific Islander, Indian, Asian, Middle Eastern, non-white African)
  • First-degree relative with diabetes
  • Obesity pre-pregnancy (BMI over 30kg/m2)
  • Fetal macrosomia in a previous pregnancy (birthweight more than 4500g or over the 90th percentile for gestational age)
  • Polycystic ovarian syndrome
  • Use of drugs that can cause hyperglycaemia (such as corticosteroids)

All pregnant women should be screened for gestational diabetes between 24-28 weeks’ gestation (unless they already have a diagnosis of diabetes) using a 75g oral glucose tolerance test (OGTT), with the preference for testing closer to 28 weeks’ gestation as testing earlier may miss some women who may develop abnormal glucose tolerance at a later date. Testing should occur earlier for those at higher risk for hyperglycaemia in pregnancy.

An oral glucose tolerance test involves the pregnant patient having their fasting blood glucose level measured, then drinking a 75g sugary drink, and then having their blood glucose levels remeasured one hour and two hours later. Diagnostic limits for gestational diabetes are listed in Table 1 below.

Table 1: Blood glucose levels in the second and third trimester of pregnancy for diagnosis of gestational diabetes (adapted from Therapeutic Guidelines: Diabetes)

Gestational diabetes
Fasting blood glucose concentration 5.1 to 6.9 mmol/L
Blood glucose concentration 1 hour after OGTT 10 mmol/L or more
Blood glucose concentration 2 hours after OGTT 8.5 to 11 mmol/L

Upon diagnosis of gestational diabetes, the patient is recommended to start a healthy eating plan and regular physical activity, whilst monitoring and maintaining their blood glucose levels within the target range whilst pregnant. Target blood glucose concentrations are listed in Table 2 below:

Table 2: Blood glucose concentration targets for gestational diabetes (adapted from Therapeutic Guidelines: Diabetes)

Second and third trimester
Fasting 4 to 5.3 mmol/L
Preprandial (before lunch and before evening meal) 4.5 to 5.5 mmol/L
1 hour postprandial 6 to 7.8 mmol/L
2 hours postprandial 5.5 to 6.7 mmol/L


Over half of pregnant women with gestational diabetes can be managed with dietary modifications and physical activity alone, and this is the preferred means of managing gestational diabetes. Optimal nutrition and controlled weight gain, with an emphasis on the quality, distribution and type of carbohydrates consumed, is critical. At least 20-30 minutes of exercise each day is also beneficial in lowering blood glucose levels. However, if diet and lifestyle modifications are insufficient in controlling blood glucose levels, then pharmacological therapy is initiated.

Therapeutic options include metformin and insulin.

It is preferable for pregnant women with gestational diabetes to use a basal-bolus insulin regime to allow for flexibility and tailored dosing. Mixed insulin regimes using fixed-dose combinations of insulin a few times a day can also be used for women who do not prefer multiple daily injections. However, this regime is more likely to cause both hyperglycaemia and hypoglycaemia.

Alternatively, metformin can be used for those who have a phobia of needles, those who refuse or have poor adherence with insulin, and those who have experienced excessive weight gain with insulin. A suitable metformin regime is:

  • Metformin immediate release 500mg once daily (or modified release once daily at night). The dose can be increased as tolerated over 1-2 weeks according to response, up to a maximum of 2000mg daily in divided doses

Specialist advice should be sought if the patient has renal impairment.

If fasting blood glucose concentrations are 5.8mmol/L or above when metformin is initiated, then monotherapy with metformin alone is usually unlikely to be effective, and insulin will mostly be required as well.

Metformin should be avoided during pregnancy if the patient is slim and has not gained enough weight during pregnancy, where there is intrauterine growth restriction, and where the pregnant woman has preeclampsia.

Delivery is usually timed around the due date (by 41 weeks’ gestation) for women whose glycaemic targets are met with diet alone and without other complications. Early delivery (i.e. around 39 weeks’ gestation) is recommended for those whose glycaemic targets are met with antihyperglycemic drugs but have no other complicating factors. Earlier delivery (i.e. around 36-38 weeks’ gestation) is recommended for those whose glycaemic targets are not met or if complications such as elevated blood pressure or intrauterine growth restriction exist.

Women with gestational diabetes should have their insulin and metformin stopped with the onset of labour or when fasting before a caesarean section. Blood glucose concentrations should be measured every two hours during labour; if above target, insulin is used to manage this. However, most women will not require insulin during labour.


  1. Diabetes [published 2021]. In: Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited; accessed {14/01/2023}.
  2. Diabetes Australia. Gestational Diabetes [Internet]. Canberra ACT: Diabetes Australia; 2022 [updated 2022; cited 2023 Jan 14].
  3. Nankervis A, Conn J. Gestational Diabetes Mellitus: Negotiating the confusion. Australian Family Physician [Internet]. 2013 Aug; 42 (8): 1.

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