Why I Declined the OGTT in Pregnancy

Firstly, this is my personal experience with a low risk pregnancy, no prior history and no family history. I am not giving medical advice – please speak to your Dr first before making any decisions regarding your unique circumstances. I’m also not down playing the seriousness of gestational diabetes (GD) [ever], but sharing my experience with you to highlight the potential problem when our current “gold standard” is applied to the entire population of pregnant women, without truly considering basic factors such as dietary intake and activity level.

The Australasian Diabetes in Pregnancy Society recommends a 50 or 75 g glucose challenge at 26-28 weeks in all pregnant women. The contents are filtered water containing 50 or 75g of glucose, food acid (330) and preservative (211, 202). Most often, blood samples are drawn twice – one to measure your baseline glucose level and one two hours after consuming the solution, to measure your glucose response (some labs also test one hour after consuming the solution as well). A GD diagnosis is given if the 2-hour blood glucose level is >8.5 mmol/L, although references ranges often differ between laboratories, are not universally accepted and there is ongoing debate about their validity. Read that again.

Did you also know that currently, due to COVID-19, low risk women are only required to test for fasting BGL and HbA1c? Interesting how it’s totally safe and fine to do so now, isn’t it? So if you are newly pregnant and low risk, you won’t be required to complete an OGTT for the time being. Of course, please complete the test if you have any risk factors or family history, as GD is very serious and does need to be diagnosed and treated diligently.

The problem with the OGTT

Firstly, when was the last time you drank 50-75g of glucose in one drink? That’s the equivalent of two to three cans of coke. How do you think your blood sugar might respond? It’s absolutely not the same as even eating 75g of carbohydrate in a meal that contains other macronutrients – it’s pure sugar with a glycaemic index of 100!

Secondly, this information has been taken directly from The Royal Australian College of General Practitioners (RACGP) website: “The OGTT results can be affected by carbohydrate intake and duration of fasting preceding the test, the time of day the test is performed and carbohydrate intake or activity during the test. During the 3 days preceding the OGTT, 150g of carbohydrate should be eaten (approximately ten 40g slices of bread per day)”.

Ten slices of bread?! So what happens if you don’t eat 150g carbohydrate/day? A false positive GD diagnosis perhaps? It is well documented that women who eat a moderately low carbohydrate and low glycaemic diet are more likely to receive a false positive diagnosis.

Even the aforementioned websites state:

  1. “OGTTs should be performed when fasting blood glucose levels are >5.5 mmol/l”, yet all women are routinely tested; and
  2. “Doctors also use fasting blood sugar and haemoglobin A1c (HbA1c) levels to evaluate diabetes”.

Let’s look at some of the research

Paper 1 – HbA1c: A Useful Screening Test for Gestational Diabetes Mellitus

  • Conclusions: Although HbA1c cannot replace OGTT in the diagnosis of GDM, it can be used as a screening test, avoiding OGTT in approximately 50% of women, if a cut off of 5.3% is used.”
  • More here: DOI: 10.1089/dia.2015.0041

Paper 2 – Screening for Gestational Diabetes Mellitus by Measuring Glycated Hemoglobin Can Reduce the Use of the Glucose Challenge Test

  • Conclusions:HbA1c can be used as a screening test prior to the GCT, thereby reducing the need for the GCT among pregnant women at a low risk of GDM.”
  • More here: DOI: 10.3343/alm.2019.39.6.524

Paper 3 – Glycosylated haemoglobin for screening and diagnosis of gestational diabetes mellitus

  • Conclusions: Our results suggest that pregnant women with an HbA1c of ≥5.4% (36 mmol/mol) should proceed with an OGTT. This may result in a significant reduction in the burden of testing on both patients and testing facility staff and resources. Further investigations are required to integrate and optimise the HbA1c as a single, non-fasting, screening tool for GDM.”
  • More here: DOI: 10.1136/bmjopen-2016-011059

Paper 4 – HbA1c for Diagnosis and Prognosis of Gestational Diabetes Mellitus

  • Conclusions: HbA1c showed high sensitivity with relatively low specificity for diagnosis of GDM in pregnant women and was a potential predictor of PDM. HbA1c may be able to be used as a simple and less invasive alternative screening test for OGTT in GDM patients.
  • More here: DOI: 10.1016/j.diabres.2015.07.014

So what does all this mean?

It is not dangerous advice to consider using HbA1c as a screening tool in trimester 1, with only those with a HbA1c ≥5.4% to proceed to OGTT. As HbA1c is a three month trend, it would make sense to test again in trimester 2 and 3, just like you would re-test your iron studies, for example. 

Please note:

  1. If your fasting blood glucose levels are ≥5.5 mmol/L, the above advice does not apply to you. But you do know how you can get your fasting blood glucose levels down now don’t you? So simply ask for a retest and go back for a repeat fasting blood glucose test once you’ve made the appropriate dietary changes. A moderately carbohydrate, low glycaemic diet is not the solution for everyone, but something to consider first.
  2. If you do get diagnosed with GD, please know that in many cases it can be managed with nutrition and exercise. The standard GD advice however, needs a little improving. Diabetes Australia still give standard advice including: “carbohydrate in every meal and snack (e.g. multigrain bread, bulgur, pasta, potato, lentils, chickpeas, beans) and low in fat, particularly saturated fat (e.g. use oils such as canola, olive and polyunsaturated oils and margarines and use lean meats such as skinless chicken and low fat dairy foods)”.
  3. If you do get diagnosed with GD, please know that your home BGL monitoring is such an important part of education and management. It will soon become very clear what meals increase your blood sugar too much, and which don’t. You may also notice that when you don’t follow the standard GD advice that your blood sugar control improves significantly. Your Dr will give you your targets, but essentially ≥7 mmol/L two hours postprandial means the food or meal was too high in CHO for you on that particular day. As exercise improves insulin sensitivity, you may also find that you respond differently to foods on the days you exercise.

It’s your body and it’s your baby. You are entitled to ask questions without being disrespected, undermined or made to feel guilty for your choice. And just because one approach is not the ‘gold standard’, does not mean it is incorrect. If you eat and move differently to the protocol used to create the ‘gold standard’, then you don’t need a degree in medicine to understand that a different protocol may be even more suitable for you.


Khalafallah A et al. 2016. Glycosylated haemoglobin for screening and diagnosis of gestational diabetes mellitus. BMJ Open, 6, 4, e011059. 

Kwon SS et al. 2015. HbA1c for Diagnosis and Prognosis of Gestational Diabetes Mellitus. Diabetes Research and Clinical Practice, 110, 1, 38-43.

Maesa JM et al. 2019. Screening for Gestational Diabetes Mellitus by Measuring Glycated Hemoglobin Can Reduce the Use of the Glucose Challenge Test. Annals of Laboratory Medicine, 39, 6, 524-529. 

Soumya S et al. 2015. HbA1c: A Useful Screening Test for Gestational Diabetes Mellitus. Diabetes Technology & Therapeutics, 17, 12, 851-853.

Further Reading

I Failed the Glucola – Lily Nichols. Available:


  1. Jodie

    Hi Steph,
    Thanks for the information, I really appreciate the time and effort you have put into this article. This test has never sat well with me. I am already into my second trimester of pregnancy, is it too late for me to do HbA1c instead? I notice above you mention 1st, 2nd and 3rd trimester screenings.
    Thank you

  2. Steph Lowe

    Hi Jodie, I’m so glad you have found it helpful. It’s definitely not too late and a really great measure to track now and in T3. Congratulations xo

Leave a Reply