Gestational Diabetes

Gestational diabetes mellitus (GDM) is diabetes that first appears during pregnancy. GDM affects approximately 10% of pregnancies, with the prevalence continuing to rise in parallel with that of obesity and type 2 diabetes.

Gestational diabetes can be associated with serious medical complications for both mother and baby. Women who experience GDM are at a much higher risk of experiencing diabetes in future pregnancies and have a 50% risk of developing type 2 diabetes later in life.

What happens in the body?

A woman’s Insulin requirements increase during pregnancy. This is due to increased food intake, maternal weight gain, and hormonal changes such as placental growth hormone, and placental lactogen, as well as increased prolactin and growth hormone production. As the pregnancy advances, the pancreatic β-cells are expected to keep up with the demand for increased insulin. Insulin resistance, the failure of the β-cell mass to expand, and a relative inadequate rise in insulin production all contribute to the development of diabetes.

Maternal glucose is transported across the placenta to the  growing foetus, and this delivery depends on the concentration gradient between the fetus and the maternal glucose levels. In the later part of pregnancy, the foetus diverts an increasing amount of maternal glucose towards itself, which leads to a decrease in maternal glucose levels. In order to maintain the concentration gradient of glucose across the placenta between the mother and the foetus, the maternal insulin resistance increases, as well as the hepatic glucose production. In turn, the β-cells increase insulin secretion to prevent excessive delivery of glucose to the foetus.

Antenatal Screening and Diagnosis of Gestational diabetes

Regular screening for diabetes during pregnancy is especially important for those women who are at high risk:

Currently guidelines from Te Whatu Ora recommend that an HbA1c test is performed initially, and this is followed up by either a Glucose Challenge Test or an Oral Glucose Tolerance Test.

Early pregnancy

Universal screening using glycated haemoglobin (HbA1c), as part of ‘booking’ antenatal blood tests (ideally before 20 weeks), will identify women with probable undiagnosed diabetes or prediabetes. Women with an HbA1c ≥ 50 mmol/mol should be under the care of a service that specialises in diabetes in pregnancy. Women with HbA1c values in the range of 41–49 mmol/mol should be offered the diagnostic oral glucose tolerance test at 24–28 weeks as they are at an increased risk of gestational diabetes. Some local policies currently treat women with HbA1c values in the range of 41–49 mmol/mol.

At 24–28 weeks’ gestation

At 24–28 weeks’ gestation, all women not previously diagnosed with diabetes who are at high risk of gestational diabetes (HbA1c of 41–49 mmol/mol) should be offered the diagnostic two-hour, 75 g oral glucose tolerance test. (If fasting glucose ≥ 5.5 mmol/L or two-hour value ≥ 9.0 mmol/L, refer to services that specialise in diabetes in pregnancy.) All other women should be offered screening for gestational diabetes using the one-hour, 50 g, oral glucose challenge test known as the polycose test. (If glucose ≥ 11.1 mmol/L, refer directly to services that specialise in diabetes in pregnancy without further testing; if glucose ≥ 7.8–11.0 mmol/L, arrange a 75 g, two-hour oral glucose tolerance test (OGTT) without delay).

You may be offered to take part in a study called the GEMS Trial. This is a randomised clinical trial looking at outcomes using different diagnostic criteria.

For further details of the New Zealand GEMS Trial contact gems@auckland.ac.nz or go to www.ligginstrials.org/GEMS

IMPORTANT NOTE: With an anticipated change in HbA1c diagnostic criteria to align New Zealand with the rest of the world the above levels are likely to be changed and New Zealand guidelines for Diabetes in Pregnancy will likely be updated.

Diabetes and Health care during Pregnancy

As with all types of diabetes, the main treatment approach is aimed at keeping blood glucose levels in a safe and healthy range. A healthy lifestyle should underpin any pregnancy, but is especially important if you have diabetes in pregnancy.

Medication and Supplements

Metformin and insulin are the only two recommended therapies for GDM. Other medicines approved for treating type 2 diabetes – such as SGLT2 inhibitors or GLP1 receptor agonists – have not yet been approved for use during pregnancy.

In addition you should be advised to stop taking some other medications that may potentially harm the baby – e.g. statins and/or ACE inhibitors or angiotensin receptor blockers.

Daily folic acid (usually 5 mg) and iodine (usually mcg) should ideally have been started from conception unless contraindicated.

You may also be offered

  • aspirin (usually 100 mg daily for prevention of preeclampsia if less than 20 weeks gestation
  • calcium supplementation for prevention of preeclampsia if low calcium diet
  • vitamin D supplementation if you have dark skin

Blood glucose management

Blood glucose levels (rather than HbA1c) are the best targets for glycaemic control in pregnancy as HbA1c often becomes unreliable due to increased red cell turnover.  Monitor blood glucose levels before meals and 1 or 2 hours post meals.

Aim for

  • Pre-meal BGL < 5 mmol/L (but be aware of hypoglycaemia)
  • 1 hour post-meal BGL < 7.4 mmol/L, or
  • 2 hour post-meal BGL < 6.7 mmol/L

In some cases you may be able to access continuous glucose monitoring (CGM) prior and/or during pregnancy. CGM can predict and detect asymptomatic hypoglycaemia and post-prandial peaks which may not be picked up using finger prick blood glucose monitoring.

>> More on Blood glucose monitoring

Potential problems during the pregnancy

Poorly managed diabetes during pregnancy can lead to serious health complications for both the mother and the baby. The risks are the same for women with pre-existing type 1 or type 2 diabetes and for those who develop gestational diabetes. Keeping blood glucose levels  in a healthy range before and during pregnancy can significantly lower these risks.

>> More on Potential Complications of Pregnancy with Diabetes

The Birth Plan

Whilst the best Birth Plans can sometimes go ‘wrong’ on the day, it’s really important that both you and your healthcare team are comfortable with YOUR Birth Plan. Think of it as a list of your preferences

Some things to consider will include:

  • Any previous birthing experiences
  • Your preferred location/environment; clinical circumstances will obviously impact this at the time
  • Pain relief
  • A birthing partner
  • Immediately after the delivery – e.g. skin-to-skin contact
  • What would you like to happen to the placenta

Caesarian Section (C-section)

Women with Type 1 diabetes have a higher rate of C-sections compared to those without diabetes. Even if it’s not your preferred method of delivering your baby, it pays to be prepared for this eventuality.

Potential complications that can lead to either a planned or emergency C-section include: 

If your C-section is scheduled, you may have the opportunity to discuss how your diabetes is managed during the procedure. Alternatively you can include your preferences in the Birth Plan.

The surgery might take up to an hour, depending on how things go. Usually you would be given an epidural (spinal block) so that you are fully conscious but can’t feel anything below your waist. A screen is positioned such that you can’t see what’s going on below, but your birthing partner can usually observe if desired.

Remember, as soon as baby is born your insulin requirements will drop back to levels close to those prior to pregnancy. This means you may no longer require treatment for diabetes.

However, close monitoring of maternal blood glucose levels is important for the first 3 months after baby is born, and annual HbA1c tests should be performed on an ongoing basis. At least 50% of women who experience GDM will go on to develop type 2 diabetes later in life.

Breastfeeding

Ideally you will be considering breastfeeding for at least the first few months if possible, but it’s not “do-or-die”…

Page updated September 2025

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