Gestational diabetes – what is it, how can I prevent it, how do I know if I have it and what is the OGTT?

What is gestational diabetes mellitus (GDM)?

Gestational diabetes is a type of diabetes that affects pregnant women. It is a condition that is characterised by high blood glucose levels (hyperglycaemia) during pregnancy, technically defined as “glucose intolerance with onset or first recognition during pregnancy” (WHO 2013, IADPSG 2010, ADIPS 2014, ADA 2001). Those diagnosed with GDM therefore include women with undiagnosed pre-existing prediabetes (which is first picked up during pregnancy), and those that develop it during pregnancy.

So what exactly does this mean?

When we eat carbohydrates, they are broken down into glucose (sugar) and absorbed into our bloodstream. In response, we release a hormone called insulin which removes the glucose from our blood and takes it to where it is needed in our cells. However, when we are pregnant, our placental hormones can “block” our insulin, preventing it from doing its job properly. This is known as insulin resistance. In a normal pregnancy, the body will combat this resistance by producing MORE insulin (about 2-3 times more than non pregnant women). However, in some women, this adaptation doesn’t go as planned, meaning women can’t keep up with the higher insulin demand. This results in elevated blood sugars or hyperglycaemia (and a diagnosis of GDM). Once the baby is born, the placental hormones stop and your insulin and glucose metabolism will return to its pre-pregnant state.


Gestational diabetes is common in Australia. Data from 2018 shows an occurrence in around 16% of pregnancies (AIHW 2020). However it is difficult to say how reliable these statistics are, as it depends on what diagnostic criteria were used. Either way, GDM is on the rise, mostly due to the changing demographic of Australians – having babies at an older age, increase rates of type 2 diabetes mellitus (T2DM), women in larger bodies and a more sedentary lifestyle.


If you have been diagnosed with gestational diabetes, you can still have a healthy baby. However it is important that your blood sugars are well managed to reduce the risk of developing complications for you and bub. Working with a team of health professionals that specialise in GDM is essential.


Can I prevent gestational diabetes?

There are some things you can do to reduce your chance of developing GDM, however in some cases, it is completely out of your control.

Those at a higher risk of developing GDM include (DA 2023):

  • Older mothers (> 35 years old). As we age, insulin resistance increases.

  • Women in larger bodies. Adipose (fat) tissue releases increased fatty acids, glycerol, hormones, pro-inflammatory cytokines and other factors that are involved in the development of insulin resistance.

  • Polycystic ovary syndrome (PCOS). PCOS is often (but not always) related to insulin resistance.

  • History of GDM or large for gestational age baby

  • Previous miscarriage or stillbirth

  • Family history of T2DM

  • Ethnic origin including Aboriginals and Torres Straight Islanders

  • Medications such as corticosteroids, antipsychotics


Maintaining a healthy lifestyle before and during pregnancy is one of the most effective ways to reduce your risk of developing gestational diabetes. This includes being active and eating a well balanced nutrient-dense diet. The main things to focus on from a dietary perspective include:

  • Removing refined sugar/carbohydrates

  • Choosing low GI carbohydrates and spreading them out over the day

  • Eating a balance of macronutrients (protein, fat and carbohydrates) at each meal and snack

  • Increasing your intake of fibre. Fibre helps to slow down the digestion of carbohydrates.

  • Ensuring adequate protein intake


Many micronutrients play a role in insulin resistance as well. Multiple studies have shown that vitamin D deficiency is correlated with gestational diabetes, and supplementation can reduce this risk (Yue and Ying 2020, Shahgheibi et al 2016, Amraei et al 2018). A good reason to ensure your vitamin D level is checked by your GP in your preconception or early pregnancy bloods so that you can correct the deficiency as soon as possible. Magnesium is another nutrient that is required for glucose utilisation and insulin signalling. Women with low magnesium levels are more likely to develop gestational diabetes  (Mostafavi et al 2015). Testing for plasma magnesium is not common (or necessarily indicative of magnesium status), but you can always increase magnesium rich foods in your diet such as nuts, seeds, green leafy vegetables, wholegrains and legumes.

Another supplement that can improve insulin resistance is myoinositol. While the focus of myoinositol research has been on polycystic ovary syndrome (PCOS) and type 2 diabetes mellitus (T2DM), there are studies suggesting that myoinositol may reduce the incidence of GDM in those who are at a greater risk (Sob 2019, D’Anna 2011, Mashayekh-Amiri et al 2022). It’s essential you get advice from your healthcare team before commencing supplements.

When it comes to exercise, it’s clear that the more you move your body the better from a blood sugar perspective. When muscles are exercised, they use glucose for energy, which results in a drop in blood sugar. Exercise can also improve your body’s sensitivity to insulin, prevent excessive weight gain and lower anxiety and stress (all of which contribute to lowering blood sugar). A systematic review from 2018 revealed that 140 minutes/week of moderate exercise was effective at lowering the risk of developing GDM (Davenport et al 2018).


Aside from diet and exercise, sleep and stress can also exacerbate insulin resistance and may increase your risk of developing GDM. While these things can be difficult to control (especially if you have young children), it is really important to try to go to bed a bit earlier, manage your stress (taking some time out for yourself and getting support) and put your phone away an hour before going to bed to help.


Please remember that even women who take all the necessary steps to prevent gestational diabetes can still develop the condition. It is NOT your fault, your body is going through many metabolic and hormonal changes and there is so much we can do to support you to manage the condition.

Diagnosing GDM – Is the OGTT the only option?

The oral glucose tolerance test (OGTT) is currently the gold standard for the diagnosis of GDM, performed at 24-28 weeks gestation. This recommendation was developed by the International Association of the Diabetes and Pregnancy Study Groups (IADPSG) and is based on the findings of the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study (IADPSG 2010, HAPO 2008).


The OGTT is thoroughly researched, inexpensive, relatively simple and most women manage the test quite well (despite needing to drink 75g of glucose on an empty stomach – that’s the equivalent of the carbohydrates in 5 pieces of toast!). However, like all diagnostic methods, there are limitations to the OGTT and it may not be appropriate for all women. Some examples include:

 – Women with hyperemesis gravidarum (these women may vomit the glucose solution)

 – Women who feel faint or dizzy when fasting for an extended period

 – If you are unable take 4 hours off work or you don’t have childcare for your toddler

 – If you usually consume a diet very low in carbohydrates, the reliability of the test is lowered

 – Women who have had a gastric sleeve (these women may not tolerate the glucose solution)


So what are your other options? If the OGTT is not appropriate for you, there are some other testing methods available.


Fasting Plasma Glucose (FPG) Test – A blood test that measures the level of glucose in the blood after an overnight fast. This test is less time-consuming and does not require women to consume the 75g glucose solution used in the OGTT. The downside to this test is that it will not provide information on your body’s ability to remove glucose from the bloodstream after consuming carbohydrates. This test is best used in conjunction with other tests.

Haemoglobin A1c (HbA1c) – HbA1c is a blood test that measures your average blood glucose level over the past 2-3 months. It can be used to diagnose GDM in early pregnancy if the value is >5.9% (Hughes et al 2014). However ADIPS reports its use for diagnosis is limited and more research is needed. Please note that HbA1c is not an accurate test if it is performed in trimester 2 or 3, as blood glucose metabolism changes throughout pregnancy.

Home blood glucose monitoring – Home blood glucose monitoring requires you to test your blood sugars regularly over the day. This can be done using a glucometer or a continuous glucose monitor (CGM). This is the most accurate way of diagnosing GDM because it gives us information on your blood sugars throughout a normal day of eating.


It is extremely important that you discuss these options with your primary health care provider and have the support of a dietitian/nutritionist who specialises in gestational diabetes. Undiagnosed GDM carries risks to both mother and child, so testing appropriately is essential.

Have you been diagnosed with gestational diabetes or want support implementing strategies to prevent it? Our experienced team of practitioners in our online clinic are here to help. Book with one of our naturopaths and nutritionists HERE.


Renee Jennings (Dietitian and Nutritionist, APD)


Luka McCabe (RN/RM/Nutrition Consultant)

Kate Holm (Naturopath & Nutritionist)


  1. World Health Organization. Diagnostic Criteria and Classification of Hyperglycaemia First Detected in Pregnancy. 2013

  2. International Association of Diabetes and Pregnancy Study Groups Consensus Panel (IADPSG), Metzger BE, Gabbe SG, et al. International association of diabetes and pregnancy study groups recommendations on the diagnosis and classification of hyperglycemia in pregnancy. Diabetes Care 2010;33(3):676–82. doi: 10.2337/dc09-1848.

  3. Australasian Diabetes in Pregnancy Society (ADIPS) Consensus Guidelines for the Testing and Diagnosis of Hyperglycaemia in Pregnancy in Australia and New Zealand, modified Nov 2014.

  4. American Diabetes Association. Clinical practice recommendations 2001: gestational diabetes mellitus. Diabetes Care 2001;24:Suppl 1:S77-S79.

  5. Australian Institute of Health and Welfare, Gestational diabetes; Figure 1: Incidence of gestational diabetes, 2000–01 to 2017–18,, 2020, accessed 1 November 2020.

  6. Diabetes Australia, Gestational Diabetes., accessed 20/03/2023

  7. Yue, CY., Ying, CM. Sufficience serum vitamin D before 20 weeks of pregnancy reduces the risk of gestational diabetes mellitus. Nutr Metab (Lond) 17, 89 (2020).

  8. Shahgheibi S, Farhadifar F, Pouya B. The effect of vitamin D supplementation on gestational diabetes in high-risk women: Results from a randomized placebo-controlled trial. J Res Med Sci. 2016 Jan 28;21:2.

  9. Amraei Mansour, Mohamadpour Safoura, Sayehmiri Kourosh, Mousavi Seyedeh Fatemeh, Shirzadpour Ehsan, Moayeri Ardeshir. Effects of Vitamin D Deficiency on Incidence Risk of Gestational Diabetes Mellitus: A Systematic Review and Meta-analysis. Frontiers in Endocrinology, vol 9, 2018.

  10. Mostafavi E, Nargesi AA, Asbagh FA, Ghazizadeh Z, Heidari B, Mirmiranpoor H, Esteghamati A, Vigneron C, Nakhjavani M. Abdominal obesity and gestational diabetes: the interactive role of magnesium. Magnes Res. 2015 Dec;28(4):116-25. doi: 10.1684/mrh.2015.0392. PMID: 26878251.

  11. Sobota-Grzeszyk A, Kuźmicki M, Szamatowicz J. Myoinositol in the Prevention of Gestational Diabetes Mellitus: Is It Sensible? J Diabetes Res. 2019 Dec 7;2019:3915253. doi: 10.1155/2019/3915253. PMID: 31886278; PMCID: PMC6925787.

  12. D’Anna R, Di Benedetto V, Rizzo P, Raffone E, Interdonato ML, Corrado F, Di Benedetto A. Myo-inositol may prevent gestational diabetes in PCOS women. Gynecol Endocrinol. 2012 Jun;28(6):440-2. doi: 10.3109/09513590.2011.633665. Epub 2011 Nov 28. PMID: 22122627.

  13. Mashayekh-Amiri, S., Mohammad-Alizadeh-Charandabi, S., Abdolalipour, S. et al. Myo-inositol supplementation for prevention of gestational diabetes mellitus in overweight and obese pregnant women: a systematic review and meta-analysis. Diabetol Metab Syndr 14, 93 (2022).

  14. Davenport MH, Ruchat S, Poitras VJ, et al. Prenatal exercise for the prevention of gestational diabetes mellitus and hypertensive disorders of pregnancy: a systematic review and meta-analysis. British Journal of Sports Medicine 2018;52:1367-1375.

  15. HAPO Study Cooperative Research Group, Metzger BE, Lowe LP, et al. Hyperglycemia and adverse pregnancy outcomes. N Engl J Med 2008;358(19):1991–2002.

  16. Ruth C.E. Hughes, M. Peter Moore, Joanna E. Gullam, Khadeeja Mohamed, Janet Rowan; An Early Pregnancy HbA1c ≥5.9% (41 mmol/mol) Is Optimal for Detecting Diabetes and Identifies Women at Increased Risk of Adverse Pregnancy Outcomes. Diabetes Care 1 November 2014; 37 (11): 2953–2959.

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