Continuous glucose monitoring in pregnancy.
Do you have gestational diabetes? Or perhaps you are a health professional who works with pregnant women. Either way, you may be interested in learning about continuous glucose monitoring (CGM) in pregnancy.
In this article you will find out what a CGM is, the difference between CGM and fingerpick testing, why CGMs are not commonly offered to women with gestational diabetes (GDM), and some research into the use of CGMs in pregnancy.
Common terms:
BGL - Blood glucose level
CGM - Continuous glucose monitor
GDM - Gestational diabetes mellitus
ISF - Interstitial fluid
T1DM - Type 1 diabetes mellitus
T2DM - Type 2 diabetes mellitus
What is a continuous glucose monitor (CGM)?
A CGM is a device that measures your glucose levels 24/7. The device (also known as a sensor) attaches to your arm or abdomen via an adhesive patch. It has a very fine needle which sits in the subcutaneous adipose tissue to access your interstitial fluid (ISF). It is not painful. Most sensors last for 2 weeks (depending on the brand), then you need to replace the sensor with a new one. The sensor is constantly reading your glucose levels, sending data wirelessly to a monitoring device that displays your readings. Some systems come with a separate monitor, however most display the information via a smartphone app.
CGM device prior to insertion. You can see the fine needle and the adhesive patch which attaches to your arm.
Post insertion. Very easy to attach and not painful at all.
How does it differ from fingerprick testing?
First of all, a CGM measures your glucose levels 24/7, as opposed to the fingerprick method which only gives you your blood glucose reading for that point in time.
Secondly, a CGM measures the glucose level in ISF, where as a fingerprick test measures the glucose level in capillary blood. ISF is the fluid that surrounds cells in the body. It's made up of water, glucose, salt, fatty acids, minerals (such as calcium, magnesium and potassium) and by-products of metabolism. Because the glucose level in ISF is not quite the same as the level in our blood, a CGM will apply a formula to give a close guideline of what our BGL is likely to be at that point in time. It is important to note that glucose levels in our blood rise and fall ahead of glucose levels in our ISF, therefore you will experience a few minutes delay in your CGM reading compared with the glucose reading given by a blood glucose meter.
Due to this delay, you may think this places people using CGMs at higher risk of experiencing hypo- or hyperglycaemic events. But this is not necessarily the case, as CGMs have inbuilt alerts and alarms for actual or impending hypo- and hyperglycaemia. They will also pick up nocturnal hypo- and hyperglycaemia (ie. low or high blood levels over night).
CGM provides information unattainable by intermittent fingerprick testing. Newer technology has made them more accurate, smaller, cheaper (however they are still quite expensive) and more user-friendly. CGM can inform, educate, motivate and alert people with diabetes.
CGM use in pregnancy - what does the research say?
Many studies have shown that using CGMs to manage diabetes in pregnancy is safe and accurate (Polsky and Garcetti 2017, Yogev et al. 2003, Chen et al. 2003, Kerssen et al. 2004).
The strongest research we have on CGM use in pregnancy is in those with pre-existing type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). A randomised control trial (RCT) in 2008 looked at glycaemic control and birth outcomes in pregnant women with T1DM or T2DM. Results found that continuous glucose monitoring during pregnancy is associated with improved glycaemic control in the third trimester, lower birth weight, and reduced risk of macrosomia (Murphy et al. 2008). Similarly, a bigger multicentre, multinational RCT, known as the CONCEPTT trial, demonstrated the benefit of CGM in addition to fingerpick testing in pregnant women. CONCEPTT found a statistically significant lower incidence of macrosomia, a reduction in neonatal hypoglycemia, a 1-day reduction in hospital length of stay, and fewer neonatal intensive care admissions (Feig et al. 2016). It's hypothesised that the increased scrutiny of glucose levels facilitated by the CGM may have contributed to increased motivation and compliance with self management in these studies.
High quality research on using CGMs to manage GDM (ie. diabetes that was diagnosed during pregnancy, rather than pre-existing diabetes) is limited. A pilot study in 2003 examined the efficacy of a CGM system in patients with GDM treated with insulin. Data from the CGM was compared to fingerpick glucose measurements. Results found that CGMs picked up high BGLs and nocturnal hypoglycaemic events that were unrecognised by intermittent blood glucose monitoring (Yogev et al. 2003). A similar, slightly larger study investigated CGM use in 57 women with GDM. CGM values strongly correlated with fingerprick measurements. CGMs better detected high postprandial glucose levels and nocturnal hypoglycemia (Chen et al. 2003). The researchers of both studies concluded that larger prospective studies are needed to determine the clinical implications of this monitoring technique.
Although more studies are needed, CGM use has promise as a therapy to improve outcomes in pregnancies associated with diabetes. Unfortunately in Australia (and most countries), they are not subsidised by the National Diabetes Service Scheme, and therefore they are quite an expense to women. Each sensor costs around $90 (depending on the brand) and you need a new sensor every 2 weeks.
Note: From experience wearing a CGM myself, and working with women using a CGM to manage their GDM, I have found that fasting BGL's tend to be a little lower on a CGM compared to a fingerprick test. For this reason, I advise women who are wearing a CGM to also do a fingerprick test when they wake up.
Why are CGMs not commonly used or recommended in pregnancy to manage GDM?
There are many barriers to using CGMs for GDM. Here are the main ones:
Cost. A CGM costs the patient/client approximately $90/fortnight, as opposed to approximately $15/fortnight for fingerprick testing.
Lack of knowledge/training of health professionals to understand and interpret CGM readings. Introduction of any new modality of therapy requires time, energy, effort, judgment, and initiative (which all require time which is often the biggest barrier).
Until recently, the accuracy of CGM had been so far inferior to those of fingerprick testing that here was increased risk of error in the clinical application of CGM values. Accuracy and precision have improved dramatically, and many health professionals are not aware of this.
References:
Polsky S, Garcetti R. CGM, Pregnancy, and Remote Monitoring. Diabetes Technol Ther. 2017 Jun;19(S3):S49-S59. doi: 10.1089/dia.2017.0023. PMID: 28585876; PMCID: PMC5467097. (Polsky and Garcetti 2017)
Yogev Y, Ben-Haroush A, Chen R, Kaplan B, Phillip M, Hod M. Continuous glucose monitoring for treatment adjustment in diabetic pregnancies--a pilot study. Diabet Med. 2003 Jul;20(7):558-62. doi: 10.1046/j.1464-5491.2003.00959.x. PMID: 12823237. (Yogev et al. 2003)
Chen R, Yogev Y, Ben-Haroush A, Jovanovic L, Hod M, Phillip M. Continuous glucose monitoring for the evaluation and improved control of gestational diabetes mellitus. J Matern Fetal Neonatal Med. 2003 Oct;14(4):256-60. doi: 10.1080/jmf.14.4.256.260. PMID: 14738172. (Chen et al. 2003)
Kerssen A, de Valk HW, Visser GH. The Continuous Glucose Monitoring System during pregnancy of women with type 1 diabetes mellitus: accuracy assessment. Diabetes Technol Ther. 2004 Oct;6(5):645-51. doi: 10.1089/dia.2004.6.645. PMID: 15628818. (Kerssen et al. 2004)
Murphy HR, Rayman G, Lewis K, et al.: Effectiveness of continuous glucose monitoring in pregnant women with diabetes: randomised clinical trial. BMJ 2008;337:a1680. (Murphy et al. 2008)
Feig DS, Asztalos E, Corcoy R, et al.: CONCEPTT: continuous glucose monitoring in women with type 1 diabetes in pregnancy trial: a multi-center, multi-national, randomized controlled trial—study protocol. BMC Pregnancy Childbirth 2016;16:167. (Feig et al. 2016)
Siegmund T, Heinemann L, Kolassa R, Thomas A. Discrepancies Between Blood Glucose and Interstitial Glucose-Technological Artifacts or Physiology: Implications for Selection of the Appropriate Therapeutic Target. J Diabetes Sci Technol. 2017 Jul;11(4):766-772. doi: 10.1177/1932296817699637. Epub 2017 Mar 21. PMID: 28322063; PMCID: PMC5588840.
