Rethinking Glucose Tolerance Testing in Active Pregnancies: Why Care, Context, and Preparation Matter

The case of a false positive test for gestational diabetes in a pregnant athlete.

Pregnancy is already full of change — physically, mentally, and emotionally. When a pregnant person is told they have gestational diabetes mellitus (GDM), it’s not just a number on a lab report. It can affect how they eat, how they move, and how safe they feel in their own body during a time when so much feels out of their control.

That’s why I want to share my story with a false GDM diagnosis. Not to criticize anyone, but to highlight how the system can unintentionally place a heavy burden on pregnant people. A misdiagnosis isn’t harmless. It can bring worry, guilt, and shame, and leave a lingering sense that your body is failing — even when it’s doing exactly what it’s supposed to do.

I’m sharing this story to help others navigate testing preparation more confidently, to provide evidence based insight, and to advocate for care that truly considers the context of each person’s pregnancy.

My Story: Two Pregnancies, Two False Alarms

First Pregnancy (2022)

During my first pregnancy, my 50g glucose challenge test (GCT) came back at 11.2 mmol/L — a value flagged as diagnostic for GDM. At my first visit with a locum physician, I requested further testing, but was denied with a blank "you have GDM", and referred to internal medicine and diabetes education.

Ironically, the day the clinic tried to reach me with the results, I missed the call because I was out on a 14 km hike. At 24 weeks pregnant, I had also recently “raced” a hilly 10 km run, averaging 4:47/km. I was surprised by the GCT result, but I wasn’t in denial; I fully understand that GDM can happen to anyone.

Here’s the twist: I didn’t have GDM.

But I wouldn’t crack the case until weeks later.

I’m a Certified Diabetes Educator, and even I didn’t immediately recognize what had happened. It took almost a week before I became certain something was wrong. During that week, I was self-monitoring my blood sugars excessively (fasting, before eating,1hr after and 2hrs after eating, before and after exercise, and bedtime) and trying to understand my patterns and make sense of the diagnosis. It started to click.. and I had my "ahhh shit" moment!

The night before my 50g GCT, I had gone for a run, had a usual supper, and then ate a very low carbohydrate breakfast and snack (intentionally..eeeek!) hoping for a lower result so I wouldn’t need the follow up 75g oral glucose tolerance test (OGTT).

PSA: never do this — more on that later!

That choice meant my glycogen stores were temporarily depleted, my first phase insulin response was delayed, and key enzymes and glucose transporters were temporarily down regulated. This is a normal metabolic adaptation to low carbohydrate intake — not diabetes. But I deeply regret eating low-carb breakfast and running the night before testing because it set me up for a falsely elevated GCT and a diagnosis of GDM.

Once I started to understand, I advocated to my usual prenatal physician (who is fantastic) for a 75 g OGTT and an HbA1c, everything became clear. My results were completely normal:

• Fasting: 4.9 mmol/L

• 1-hour: 5.0 mmol/L

• 2-hour: 5.7 mmol/L

• HbA1c: 4.8%

If I hadn’t pushed for repeat testing, I would have been labelled high risk, underwent unnecessary interventions and taken on enormous psychological stress — all with real potential for harm.

And honestly, I still get emotional thinking about what could have happened. With a GCT of 11.2 and the one hour (not 2 hour) post-prandial blood sugars I was seeing (>6.7 mmol/L), while I was restricting carbohydrates (trying to “manage” a diagnosis I didn’t actually have), insulin would not have been an unreasonable next step. The issue wasn’t uncontrolled glucose — it was my carb intake. It wasn’t until I increased to >100 g of carbs per meal that my one hour post prandial readings normalized. My mental health improved immensely after I went back to my usual high carb diet, and I got back to usual exercise because I finally felt fantastic again.. because I was FED!

I’m incredibly grateful for my nutrition and diabetes training, my ability to experiment with nutrition safely, and the confidence to advocate for further testing. Without that, this story could have ended very differently.

Now, onto round 2..

Second Pregnancy (2025)

This time, I prepared carefully as per Klein et al., 2021 and Rosenberg et al., 2021

You can view my prep video here!

• >150g carbohydrates/day for 3 days prior

• >50g carbohydrates in the meal before the GCT

• Avoided moderate-to-high intensity exercise for >48hrs prior to testing.

  
  

Even then, my 50g GCT came back 8.6 mmol/L, still flagged as high and recommended further testing with a 75g OGTT. So I went above the recommendations and consumed 350g of carbs (~5g/kg) the day before, and...

The 75g OGTT, was entirely normal.. AGAIN:

• Fasting: 4.4 mmol/L

• 1-hour: 4.2 mmol/L

• 2-hour: 5.0 mmol/L

• HbA1c: 4.9%

Again, not GDM. Just a metabolically flexible, healthy pregnant athlete doing exactly what her body was designed to do.

For context, this is from the Canadian Diabetes Association Clinical Practice Guidelines:

Why These Results Happen

Short term nutrition and exercise can significantly affect glucose tolerance, and pregnant athletes are not exempt.

Low carbohydrate availability, even for a single meal, can delay insulin secretion and blunt glucose uptake. This can make post meal glucose values appear higher than they truly are, especially in active individuals meeting energy needs, who are already primed to store carbohydrates efficiently..

Recent endurance exercise has a similar effect, and if you've ever been pregnant, you know the second trimester is when you feel the best, hence the highest chance for regular exercise! I tend to lose fitness in the first trimester, have a slight gain in the second, and then a gradual decline until the end, extending until at least 2 months postpartum. The glucose tolerance tests just so happen to also be in the second trimester, also when a pregnant person may be training the most!

Honestly, I can’t help but giggle at the inaccuracy.. thinking that even now, not pregnant, if I went for a run the night before a glucose tolerance test, I’d probably meet criteria for type 2 diabetes. Luckily, glucose tolerance tests aren’t used as screening tools outside of pregnancy — because we already know they can misclassify people.

At a physiological level, key glucose transporters and enzymes involved in glucose uptake become temporarily down regulated when glycogen stores are low. As a Sport Dietitian, I teach these adaptations all the time (in simpler terms) when talking about low energy availability and REDS. As a Diabetes Educator though, it somehow didn’t click right away that these very normal, acute, expected exercise and diet induced shifts could completely distort a glucose tolerance test.

In short: a temporary, physiologic insulin-resistant-like state is not diabetes. But without context, it can absolutely look like it. And let’s be real: standard GDM testing kinda assumes pregnant folks are… sedentary.

What the Research Tells Us

In non-pregnant endurance athletes, this is documented:

• Low carbohydrate intake (even briefly) reduces first-phase insulin response and slows glucose clearance.

• Recent endurance exercise can temporarily impair glucose tolerance despite perfectly normal fasting glucose and HbA1c.

• Glycogen depletion creates a transient insulin-resistant phenotype which is reversible, predictable, and not pathological.

• Glucose tolerance adapts to recent fuel availability, not just underlying metabolic health.

We’ve known this for years. Yet standard GDM screening protocols still do not account for recent diet or training load. Usually pregnant folks are stressed NOT to change anything before glucose tolerance tests, but they should absolutely be advised on diet and exercise for ACCURATE results.. more to come...stay with me!

This isn’t just a “pregnant athlete problem.” The physiology shown in the Rosenberg et al. (2021) study applies to any pregnant individual: eating fewer carbohydrates before a glucose tolerance test can make your results look artificially high - there is an inverse linear relationship between previous days carb intake and glucose tolerance results! In the study, even a modest drop in carb intake the day before a 75 g OGTT significantly worsened 30, 60, and 120 minute glucose values. This happens because low carbohydrate availability temporarily blunts the body’s insulin response and slows glucose clearance, again a normal adaptation, not disease. So whether you’re pregnant, active, or neither, going into an OGTT with low glycogen or a lower-carb meal can lead to a false abnormal result simply because of what you ate the day before.

I've had the conversation with MANY certified diabetes educators and there is unanimous agreement that this was unknown to us and not in the study material we purchased while training to become certified diabetes educators. Each of us can think of multiple clients (e.g., active pregnancies, disordered eating, food insecurity, ++NAUSEA, etc). who likely got misdiagnosed over the years...

The Real Impact of a False Positive GDM Diagnosis

Misdiagnosis carries real consequences beyond unnecessary medical interventions:

Emotional & Psychological

Benton et al., 2023 captured this perfectly:

“It feels like medically promoted disordered eating.”

Pregnant people diagnosed with GDM can experience guilt, fear, and shame, and emotions can linger long after pregnancy. I experienced this firsthand during the week I went low-carb in an attempt to “manage” a GDM diagnosis I didn’t actually have. I was monitoring my blood sugars frequently, second guessing every meal, feeling bad about attending a family pizza party, and restricting carbs — and it affected me immediately. I do not have anxiety, depression, disordered eating or the like, but that week I got a glimpse of what it likely feels like, when the medical system didn't understand me, and after reading "unwell women" by Elinor Cleghorn, I thought to myself - this must be the "hysteria" so many women have experienced? I wasn't me mentally or physically...

Physically, I couldn’t even run 1 km. I told myself, “This is the point in pregnancy where I stop,” because it simply didn’t feel good. I was fine with that because people had warned me this may come. Mentally, I was anxious, fatigued, and frustrated - not my usual self. But the moment I returned to my normal, high-carb diet, everything changed. My mental state lifted almost instantly, and my energy returned quickly. I was able to run 10 km with ease, and at 31 weeks pregnant, I even “raced” an 8 km with a friend who was 4 months postpartum — running under 5 min/km pace.

This experience reinforced a simple but powerful truth: we perform better mentally and physically when meeting carbohydrate recommendations. Nutrition isn’t about chasing numbers on a lab result — it’s fuel for both body and mind.

Clinical & Financial

·       Extra appointments/time away from work/travel

·       Unnecessary glucose monitoring

·       Potential medication

·       Increased healthcare costs

Trust in Care

False positives erode confidence in providers and the healthcare system, and rightly so.

How We Can Reduce Unnecessary Burden

The goal is not to change who we test — it’s to make testing more accurate, safer, and less stressful. I have some solution oriented thoughts..

1. Add HbA1c + Fasting Glucose to Routine Prenatal Labs

·       Early markers can help stratify risk and guide the need for further testing.

·       Already in use in some countries.

·       Helps reduce unnecessary OGTTs and the emotional burden associated with them.

2. Standardize Pre-Test Preparation

To improve accuracy:

For most people:

·       Eat ≥150g carbs/day for 3 days

·       Eat ≥50g carbs in the meal before testing

·       Avoid moderate-to-high intensity exercise 48–72 hours prior

For athletes or highly active individuals, consider the following:

·       Avoid aerobic exercise for at least 3 days prior

·       Aim for 4–5 g/kg carbs/day for 3 days before testing

·       Include some low fiber, easily digestible carbs (juice, muffins, chocolate milk, fruit purée)

·       Request HbA1c as part of first trimester and 24–28 week labs

3. Use Clinical Context

Numbers alone rarely tell the whole story. I had a locum provider who didn't take time to listen to what I had to say, and denied retesting me after I was diagnosed with GDM in my first pregnancy.. but luckily I contacted my usual prenatal doctor and they were happy to retest!

Consider:

·       Training logs

·       Diet history

·       A1c trends

·       Home glucose monitoring.. cautiously (see below).

·       Signs of low energy availability

A caution about using glucometers in people with normal glycemia (mis diagnosed with GDM)

I validated my brand new glucometer in the lab during my 75 g OGTT — and it was off by 55% and 40%, reading significantly higher than the lab. Glucometers are supposed to fall within ±20% accuracy, and in my clinical work with people with type 2 diabetes, that’s almost always been true. So why not with me?

My results:

• 1-hour: Glucometer 7.8 vs. lab 5.0

• 2-hour: Glucometer 7.9 vs. lab 5.7

  
  

And here’s where it gets even more cringe. During that week when I thought I was “managing” my non-GDM, GDM, many of my glucometer readings were in the 4.0s. In retrospect, those were likely 40–50% higher than what was actually happening (AHHHHH). Which, of course, explains exactly why I felt so depleted — emotionally and physically. I wasn’t just restricting carbs; I was likely dealing with much lower blood sugars than I realized...

It raises an important question: Are glucometers as accurate in active people with normal blood sugars? And how many others might be interpreting misleading numbers in already stressful moments?

4. Recognize Metabolic Diversity

Pregnant people are not metabolically uniform:

·       Whole food eaters, habitual lower carb diet, and athletes process glucose differently.

·       Transient insulin adaptations don’t equal disease.

Why My Story Isn’t the Norm (And Why That Scares Me)

Here’s the part that sits heavy for me: not everyone will be this lucky. I had an unusual level of protection that most people don’t. I have years of training in metabolism and physiology. I understand (deeply), how the body responds to acute and chronic dietary changes. I’ve managed GDM in hundreds of patients. I’ve adjusted insulin independently under medical directives. I wasn’t afraid of the diagnosis. Because of my background, I knew how to experiment safely, how to interpret my own numbers, and how to advocate for the right testing. I had the confidence to question the process. I had the knowledge to pick up on what didn’t make sense. But not everyone does..

Since sharing my experience of being misdiagnosed with GDM — and later discovering a falsely elevated 50g GCT again in my second pregnancy — I’ve had countless women reach out with nearly identical stories. Endurance athletes, health conscious folks, and women who went low carb for a few days to “be healthy.” Women who were terrified when they saw the result but didn’t know what to question. Their stories are what pushed me to write this blog.

Because how can a test so heavily influenced by what we eat and do the day before be diagnostic in pregnancy?!!

Care shouldn’t depend on luck, expertise, or insider knowledge. And that’s what worries me.

So I keep thinking… what about everyone else?

1. How many endurance athletes training during pregnancy get labeled with GDM and respond by exercising as punishment—trying to “earn” normal blood sugars—rather than moving for joy, confidence, and health?

   
   

2. How many are induced early because an imprecise fetal growth ultrasound suggests a “big baby,” even though people with GDM can and often do have completely normal-weight babies… only to deliver a 5, 6, or 7-lb newborn who was never actually at risk?

   
   

3. How many pregnant people already struggling with disordered eating, carb avoidance, or active eating disorders receive a false positive GDM diagnosis—only to have their entire pregnancy overshadowed by fear, shame, and hyper-vigilance around food?

   
   

My heart honestly goes out to every single one of you. (Big hug.) If you see yourself in any of these scenarios, I would love to hear your story. I want to learn from you. And if enough people come forward, I may consider publishing a case series to formally propose updates to our GDM screening guidelines—because lived experience matters, and your stories deserve to be heard.

A Note on Care and Compassion

This isn’t about criticizing the system or providers. It’s about reducing unnecessary burden on people who can become pregnant and preventing the psychological and emotional impact of misdiagnosis. Pregnancy is a vulnerable, transformative time — every extra label or intervention carries weight.

We can provide more supportive, individualized care simply by:

• Understanding the physiology of active and metabolically flexible pregnant individuals

• Interpreting tests in context

• Preparing people properly before testing

• Including early markers like fasting glucose and HbA1c

A little knowledge and preparation can prevent weeks of stress, finger pricks, extra appointments, and guilt — all without compromising care.

Final Thoughts

Pregnancy testing doesn’t have to be a source of unnecessary worry or psychological stress. By considering context, preparation, and physiology, we can prevent misdiagnosis and reduce the emotional burden for pregnant people — all while still protecting maternal and fetal health.

Pregnant people are doing their best. Their bodies are doing their best. Let’s make sure our screening methods respect both.

P.S. I gave birth to two healthy, full-term babies — 7 lbs 3 oz with my first and 7 lbs 11 oz with my second. They are my pride and joy!

Klein KR, Walker CP, McFerren AL, Huffman H, Frohlich F, Buse JB. Carbohydrate Intake Prior to Oral Glucose Tolerance Testing. J Endocr Soc. 2021;5(5):bvab049.

Benton M, Silverio SA, Ismail K. “It feels like medically promoted disordered eating”: The psychosocial impact of gestational diabetes mellitus in the perinatal period. PLoS One. 2023;18(7):e0288395.

Conn JW. Interpretation of the glucose tolerance test. The necessity of a standard preparatory diet. Am J Med Sci. 1940;199:555-564.

Rosenberg EA, Seely EW, James K, Arenas J, Callahan MJ, Cayford M, Nelson S, Bernstein SN, Thadhani R, Powe CE. Relationship between carbohydrate intake and oral glucose tolerance test results among pregnant women. Diabetes Res Clin Pract. 2021 Jun;176:108869. doi: 10.1016/j.diabres.2021.108869. Epub 2021 May 23. PMID: 34029622; PMCID: PMC8544918.