Southern Cross Medical Library

Southern Cross Medical Library

Diabetes is when levels of glucose in your blood (blood sugar) are too high because your body has insufficient insulin or resists the effects of insulin. Gestational diabetes is a variation of the disease that occurs during pregnancy.

Gestational diabetes may not present obvious symptoms but may be diagnosed during routine pregnancy screening. The condition can adversely affect the pregnancy and health of the baby but can be managed with a healthy diet and exercise and, if necessary, medication.

General information

Diabetes mellitus (commonly referred to as diabetes) is a group of diseases characterised by high blood glucose (sugar) levels over a prolonged period of time. This page deals with gestational diabetes. Other diabetes variations include type 1 diabetes (usually diagnosed in childhood or adolescence) and type 2 diabetes (associated with a person being overweight).

Gestational diabetes is common, having been estimated to occur in 6% (about one in 17) of all pregnancies in New Zealand. An increase in the incidence of gestational diabetes in recent years appears to be related to increasing rates of obesity in the New Zealand population.


The exact cause of gestational diabetes is not known. However, pregnancy does affect how the body metabolises (breaks down) glucose. Glucose is absorbed into the bloodstream following a meal. The body then uses insulin, a hormone produced by the pancreas gland, to promote the uptake of glucose into cells so that it can be broken down and used as an energy source. In doing so, insulin lowers the level of glucose in the blood.

During pregnancy, the placenta produces an excess of certain hormones that can block the effects of insulin (insulin resistance). The reduced insulin activity leads to less efficient uptake of glucose into cells and hence the build-up of glucose in the blood. Usually, the pancreas compensates by increasing its insulin production during pregnancy (to about three or four times the normal amount). However, if the pancreas cannot increase insulin production sufficiently to compensate for the loss of insulin activity, blood glucose levels rise and cause gestational diabetes.

Risk factors

It is not understood why some women develop gestational diabetes and others do not. However, certain factors increase a woman’s risk of the developing the condition. These risk factors include:

  • increasing age (especially women aged 35 years or older)
  • family history (having a parent or sibling with diabetes)
  • previous history of gestational diabetes
  • previous history of prediabetes (a slightly elevated blood glucose level that may precede the development of type 2 diabetes)
  • having previously given birth to a large baby (more than 4kg)
  • excess weight.
  • ethnicity - women who are Māori, Pasifika, or Asian Indian are more likely to develop gestational diabetes than women who are New Zealand European
  • polycystic ovary syndrome.

Signs and symptoms

Gestational diabetes does not cause obvious signs or symptoms in most women. It is usually diagnosed following routine screening during pregnancy. It can develop at any stage of pregnancy but usually occurs in the second or third trimester. Women who are diagnosed in early pregnancy may have underlying diabetes that has not been previously recognised.


An HbA1c (glycated haemoglobin) test is obtained with the first blood test in pregnancy (‘antenatal bloods’) to determine if a pregnant woman is at risk of developing gestational diabetes.

It is recommended that all pregnant women should be tested for diabetes during the second trimester, i.e., at weeks 24–28 of pregnancy.

A two-step process to test for gestational diabetes is usually used:

1. Oral (1-hour) glucose challenge (polycose) test: a blood sample is taken one hour after drinking a glucose solution. If the result is higher than normal, a second test is recommended.

2. Oral (3-hour) glucose tolerance test: after an overnight fast, a blood sample is taken before drinking a solution containing a higher concentration of glucose and again every hour for three hours after. Gestational diabetes is indicated if at least two of the blood glucose level tests are higher than normal.

Women with risk factors for gestational diabetes may be tested earlier in pregnancy, probably at their first ante-natal visit.

Once a diagnosis of gestational diabetes has been made, continued glucose control testing and testing for diabetic complications is recommended for the remainder of the pregnancy. This may include self-monitoring as part of an overall management plan. Tests to monitor the baby’s health during pregnancy may also be done.

A follow-up Hb1Ac test will likely be done after delivery and again at 10–12 weeks to ensure that levels have returned to normal. Levels return to normal in most women, but they should still have tests for diabetes and prediabetes at least every three years.


Gestational diabetes that is not carefully managed can result in uncontrolled blood glucose levels, which may cause problems for pregnant mothers and their baby.

Complications affecting babies born to mothers with gestational diabetes include:

  • excessive birth weight. High levels of glucose in the mother’s bloodstream can trigger the baby to produce high levels of insulin, potentially causing the baby to grow larger than is normal (macrosomia). Large babies are at higher risk of problematic delivery
  • a higher risk of early (premature) birth
  • respiratory distress syndrome, especially if they are born early. Respiratory distress syndrome is a condition that makes breathing difficult
  • low blood glucose (hypoglycaemia) may develop soon after birth because the baby’s own insulin production is high. Treatment to normalise the baby's blood glucose level may be necessary
  • a higher risk of type 2 diabetes later in life.

Complications affecting mothers with gestational diabetes include:

  • pre-eclampsia, which is a serious form of high blood pressure that only occurs during pregnancy. It can be life-threatening for both mother and baby
  • a higher risk of developing gestational diabetes during a future pregnancy
  • a higher risk of developing type 2 diabetes later in life
  • Caesarean section is more likely if the baby is large
  • induction of labour if the baby is growing too big
  • polyhydramnios, which is the production of too much (amniotic) fluid around the baby. It can lead to preterm delivery.

Keeping blood glucose levels within the normal range reduces the risk of complications and increases the likelihood of a straightforward pregnancy and delivery.

Treatment and lifestyle changes

The aim of treatment and lifestyle changes is to keep the mother’s blood glucose levels within normal healthy limits.

Blood glucose level self-monitoring

This involves checking and recording blood glucose levels four or five times per day (on waking up in the morning and after meals) to ensure that they stay within their normal range. Self-checking of blood sugar levels is usually done with a finger-prick test (pricking your finger with a small sharp-pointed device called a lancet and putting a drop of blood on a test strip, which is read by a blood glucose meter). However, glucose monitoring devices that monitor glucose levels without the need for finger pricks are now available.

Post-pregnancy follow-up blood glucose level checks are also important as gestational diabetes increases the risk of developing type 2 diabetes.


A high-fibre low-fat diet based on fruits, vegetables, and whole grains is recommended to reduce the risk of diabetes. As it is not advisable to lose weight during pregnancy, weight gain goals may be set by a doctor and a meal plan developed by a dietician.

Physical activity

Physical activity helps the body to control blood glucose levels. Moderately intense exercise for 30 minutes a day on most days of the week is generally recommended. Walking, cycling, and swimming/water walking are suitable forms of exercise during pregnancy.


Although some women with gestational diabetes can maintain normal blood glucose levels with diet and exercise alone, others will need insulin injections and possibly the addition of an oral diabetes medication (e.g., metformin). Medication is stopped after delivery of the baby.


Adopting healthy habits before pregnancy will help to reduce the risk of gestational diabetes. These include:

  • eating healthy foods
  • keeping physically active
  • maintaining bodyweight within the normal BMI (body mass index) range
  • getting prenatal care early and having regular check-ups.

Further information and support

For further information and support contact your doctor, practice nurse, or any of the following organizations.

Diabetes New Zealand

Freephone: 0800 DIABETES (0800 342 238)

Email: [email protected]



Chepulis, L., Morison, B., Lawrenson, R., et al. Prevalence of gestational diabetes in the Waikato region of New Zealand. Intern Med J 2022; 52(6):1075-1078 Diabetes in Pregnancy Staff (2021). Diabetes in pregnancy: gestational diabetes mellitus (GDM) (Booklet PDF). Auckland: Women’s Health – Diabetes in Pregnancy Clinic, Auckland City Hospital. Ministry of Health Staff (2014).

Screening, diagnosis and management of gestational diabetes in New Zealand: A clinical practice guideline (PDF). Wellington: New Zealand Ministry of Health. Moore, T.R. (2022). Medscape drugs and diseases: Diabetes mellitus and pregnancy (Web Page). New York, NY: WebMD LLC. [Accessed: 06/12/22]

Mayo Clinic Staff (2022). Gestational diabetes (Web Page). Rochester, MN: Mayo Foundation for Medical Education and Research. [Accessed: 06/12/22]

Last reviewed: September 2023

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The purpose of the Southern Cross Medical Library is to provide information of a general nature to help you better understand certain medical conditions. Always seek specific medical advice for treatment appropriate to you. This information is not intended to relate specifically to insurance or healthcare services provided by Southern Cross.