Gestational diabetes: update on screening, diagnosis and maternal management

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Gestational diabetes is a common condition affecting about 14% of pregnancies. All women in Australia should be screened for gestational diabetes because of the benefits that a timely diagnosis and treatment have for both maternal and fetal health. However, who should be screened and when and how has just been updated. I'm Dr Laura Beaton. I'm a GP in Melbourne and your host for this episode.

Today, I'm joined by endocrinologist Professor Chris Nolan, who along with colleagues, has written an article for Australian Prescriber that explains the new Australasian consensus recommendations on the screening and diagnosis of gestational diabetes, and they've also provided a timely reminder for maternal management. Chris, thank you so much for speaking with me today and helping to get the word out about these new consensus recommendations for gestational diabetes.

Thank you, Laura. Pleased to be here.

Do you mind if we start off with probably the biggest change in these recommendations, the higher diagnostic thresholds for diagnosing gestational diabetes? Why has this changed and why is it such a big update?

Yes, it's a major update and the Australasian Diabetes in Pregnancy Society are really leading the way internationally with these changes in diagnostic criteria. One of the issues is not to over-diagnose and over-treat gestational diabetes, and there was some evidence from the TOBOGM [treatment of booking gestational diabetes mellitus] study that diagnosing at too low glucose may actually increase the risk of small-for-dates [small-for-gestational-age] babies. And so, we don't want to cause harm by over-treating. So, these new guidelines are suggesting higher cut points.

I guess fellow general practitioners will be very used to organising the pregnancy oral glucose tolerance test at about 24 to 28 weeks, and that recommendation has still stayed the same. What else is essentially the same in these new recommendations?

We're going through a phase of re-looking at gestational diabetes and whether we have the paradigm right or wrong, choosing to test diagnose at 24 to 28 weeks. We know that, within the Australian population and internationally, many women of childbearing age already have pre-diabetes or diabetes before they get pregnant. And if we wait till late in pregnancy to do a glucose tolerance test, we actually miss those women, at worst, the ones with pre-existing diabetes before pregnancy to miss those. There's a high rate of adverse pregnancy outcomes in that group, but there's all those women with pre-diabetes, so impaired fasting glucose, impaired glucose tolerance before they get pregnant.

They really have abnormal glucose metabolism early in pregnancy and we don't want to miss those. So, whether we should move the whole paradigm to early and do all our testing early in pregnancy, we haven't quite gone that far. But women at risk, we're suggesting that they have early screening for gestational diabetes, women that have had previous gestational diabetes to just go straight on to a glucose tolerance test. Women that have an elevated haemoglobin A1c [HbA1c] in the range of 6.0 to 6.4, so not diagnostic of overt diabetes, suggesting they have glucose tolerance tests as well. And so, all these women at risk of gestational diabetes should have early screening, at least with a haemoglobin A1c.

And this early screening definitely sits in the territory of general practice. And for those of you who are reading along at home, in the article Box 1 goes through many of the risk factors that indicate that really we should be doing some early screening for gestational diabetes, that's in that very first trimester. And actually looking at all these risk factors, many Australian pregnant women probably fall into the category that they should be getting early screening in that first trimester.

Yes, if we look at the prevalence of pre-diabetes or impaired glucose tolerance, impaired fasting glucose, we're talking about rates of around 18% within our population. So, it's not surprising that many have those risk factors and higher maternal age, higher BMI, family history of diabetes, previous gestational diabetes, all of those are very common. Not included in the ADIPS [Australasian Diabetes in Pregnancy Society] consensus statement was ethnicity because we know that many ethnic groups that are living in Australia have higher rates of gestational diabetes and type 2 diabetes, and that's another consideration for GPs within the particular population they're dealing with. Whether it's a group that's at higher risk, that's something additional to think about.

There's always something additional to think about in general practice, but that's why it's nice to have some really great summary articles to be able to reference back and bookmark. So, thank you for that. Before we get into the details of the screening and diagnostic criteria, can we just briefly step back and remind ourselves of, I guess, the risks that we're trying to avoid, both maternal and fetal, by getting a timely diagnosis and treatment? What are the risks of really under-treatment or under-detecting that we're trying to avoid?

Yeah, so essentially mothers have elevated glycaemia through pregnancy. That higher glucose level crosses the placenta to the baby and essentially causes accelerated growth of the baby, and the baby's developed hyperinsulinemia and at birth at risk of neonatal hypoglycaemia. And to have big babies, bigger babies are harder to deliver, higher risk of maternal perineal trauma, higher risk of things like shoulder dystocia, which is really a complication to avoid. With that, mums are also at higher risk of pre-eclampsia, so hypertensive disorders in pregnancy and treatment of gestational diabetes reduces that risk, but both mums and babies are at higher risk into the future of metabolic diseases.

So, overweight, obesity, diabetes, cardiovascular disease. So, it's worthwhile to diagnose and reduce those early risks of pregnancy outcome risks, but also to flag mums and their children at risk of metabolic disease later in life for preventive strategies.

Yeah, pregnancy care is one of those high touch points in the medical system that actually you get a really good chance to intervene for preventive health strategies for a woman that will actually probably carry through for the rest of her life, which is one of the benefits of such intensive management in pregnancy. What are these new cut-offs for making a diagnosis of gestational diabetes?

For the diagnosis of GDM, the fasting glucose level has been increased from 5.1 to 5.3, the 1 hour from 10.0 to 10.6 and the 2-hour cut point from 8.5 to 9.0, all of those values, the fasting 1 hour and 2 hour levels have been raised. And interestingly, they're the levels that were shown through the HAPO [Hyperglycemia and Adverse Pregnancy Outcome] study, a huge observational study, was shown to have a 2-fold increased risk in adverse outcomes of large-for-dates [large-for-gestational-age] babies, neonatal hypoglycaemia, and caesarean sections.

And for those of you who find numbers particularly hard to remember, I would recommend bookmarking Table 1 from this article, which goes through all of those glycaemic parameters for us. It also has a handy table reminding us about the overt diabetes in pregnancy. It's actually the same criteria as diabetes in non-pregnant adults, so that's a helpful one to have as well. When we consider gestational diabetes, like all diabetes, it's actually quite intensive to manage and it's actually very well suited to a coordinated multidisciplinary approach. So, can we talk through patient education and lifestyle before we talk about medicines?

Because I'm specifically interested in the kind of advice we would give to someone who is pregnant that might be different to someone who isn't pregnant with a recent diagnosis of diabetes.

So, education of the women is really the most important here I think, and to ensure that the women are having a healthy lifestyle, bringing some gentle exercise into their program, but also healthy diet, particularly avoiding the junk foods and large amounts of takeout foods that are often associated with high calorie input. So, with good dietary management, a large number of women will achieve satisfactory glycaemic values. We tend to do home blood glucose monitoring for our women and 4 tests a day before breakfast and most often 2 hours after breakfast, after lunch and after dinner. So, 4 tests a day, and if those levels are within target, then we carry on with the lifestyle change.

If we're above those targets, a review of what's happening with diet often helps and you can again get control just with lifestyle change, but sometimes we need medical therapy such as insulin and some people in Australia using metformin to manage the glycaemia. Lifestyle education is really very important and that needs to be culturally relevant because in Australia, we have women from many cultural backgrounds and their diets and foods that they're comfortable with vary tremendously, and to have management that's appropriate for their culture is really important and that's where dietitians are really helpful because they understand this.

High-quality multidisciplinary care is valuable for diabetes of any type, and I have found a good quality dietitian who's able to really look at the trends of what's really happening in that detailed glucose data is really helpful. It is pretty intensive BSL [blood sugar level] monitoring for someone who previously maybe has been well and had not very much medical input. And for those who can afford it, continuous glucose monitoring has been pretty invaluable for reducing the medical burden for gestational diabetes. What have you found in your practice?

So, we haven't been using continuous glucose monitoring (CGM) in gestational diabetes management. One of the issues for it is we don't know what glucose levels to be aiming for with CGM. The actual measurements you get are not quite the same as what you get with blood glucose, so using the same numbers may not be useful. We find that women with gestational diabetes, there's a time in range that we use for women with type 1 diabetes outside of pregnancy. Those time in range values about over 90% of women with GDM will be reaching those time in range, and it might actually make them feel they're achieving good control when they're actually not. So, we're not quite ready.

I think it will come that continuous glucose monitoring will be an option for gestational diabetes, but we're not there yet. We don't have enough evidence to know what values we need to be aiming for.

This is one of the points you also mentioned in your article around glycaemic targets. There's a reasonable amount of local variance in these fasting and postprandial blood glucose targets in gestational diabetes around the country. In the article, you do reference some targets, which I think are from the American Diabetes Association. Are these generally more or less conservative than most of the Australian local targets?

They're tighter than many targets that are used by jurisdictions around the country. Our issue with this is that we really don't have high-quality evidence of what the target should be. There have been some studies, but with not huge numbers of people and to relate that back to the many different ethnic groups and situations, we don't quite have the evidence. So, those targets are the American Diabetes Association targets. The tighter end in Australia, many people would have targets fasting of 5.3 and a 2 hour of 7.0, rather than less than 6.7 for 2 hours. So, that's really on the tight end. So, many diabetes services that do diabetes and pregnancy around the country will have their favoured targets.

They're all within reasonable close arrangements, but you can imagine a 2 hour of 6.7. If you're using that as a target to decide whether to use insulin therapy or not, you'll finish up with a higher percentage of women on insulin therapy and is that what we want? At 7.0 for a 2 hour, we'll have less people starting on insulin therapy, so the targets do make a difference.

You mentioned the lifestyle modifications and especially diet does seem to be enough for a good proportion, maybe just about a third of people with gestational diabetes, but about two-thirds are going to need to progress to medication at some point, and insulin is the mainstay of pharmacological management as you mentioned. At this stage, we've got an obstetric medicine physician or an endocrinologist involved. Are there differences in the insulin choices or regimens that we would use in gestational diabetes compared to what say GPs would be used to for insulin and non-pregnant people with diabetes?

So, in gestational diabetes, we really need to individualise insulin therapy for the particular pregnant mother. Some have problems of the fasting glucose. And to control that with insulin, a longer acting insulin at nighttime, so on going to bed is the best to control that. Others will have normal fasting sugars, but it will be blood sugars after meals that go up. And after trying to sort out the dietary reasons for that, if they're still elevated, then a rapid acting insulin before meals is the best way to go. And it might be only the lunch and the dinner ones that are elevated, and so you treat the lunch and dinner time points with insulin. Sometimes, we use insulin just if they're having particular types of meals.

Some meals, they might find the glucose is fine, but if they have a higher carbohydrate meal, they need insulin for that. Some women need both long-acting and rapid-acting insulin, so it's really individualising for the glucose profile that they have.

And in centres that manage gestational diabetes, is this dose titration normally done by the endocrinologist or obstetric medicine doctor by diabetes nurse educators who are specialists in this area, by a bit of both? How does it tend to work?

So, the models of care for managing gestational diabetes around Australia vary tremendously, and it really depends on the resources of the particular sites. It's a multidisciplinary team with diabetes nursing and dietitians, with endocrinology being available and there'll be obstetric care available. For women that are managing well with just diet and exercise and their glycaemic is well controlled, they won't need to see an endocrinologist. If they're not travelling well, it might be a nurse practitioner that starts insulin therapy. It might be a referral to an endocrinologist to do that, and that varies from site to site depending on the local resources.

We'd like to have the obstetric team involved in managing glycaemia as well. And general practitioners could also be involved in this as well. It's really understanding what the targets are, which are different than managing diabetes outside of pregnancy. They're much tighter. But once you know what you're aiming for, many healthcare professionals have the skills that will be able to manage it well.

And what about other medications beyond insulin?

Metformin is the one that is used in managing gestational diabetes. It's quite variable around the country as well. Some centres use metformin quite freely. Others save it for more complex diabetes and pregnancy. So, there's some really good studies of metformin use in gestational diabetes, which show that pregnancy outcomes are just as good and sometimes with less large-for-dates [large-for-gestational-age] babies and neonatal hypoglycaemia. Many pregnancies, metformin is not enough, so you need metformin and insulin. The real question is what are the long-term outcomes of using metformin in pregnancy? And we don't have enough data. So, we know that metformin easily crosses the placenta, so the developing baby is seeing metformin.

Metformin acts somehow on mitochondrial function. Are we really sure that metformin during pregnancy is setting the baby up for good health in the long-term? One of the reasons we treat gestational diabetes is for long-term health of the offspring. We don't quite have the evidence yet to say that it's going to be good or going to be harmful long term for the baby. We really don't know. And so, some centres are less favourable to using metformin in a lot of their pregnancies, but those risks are probably low of using metformin. Certainly, as far as women they prefer a tablet to insulin injections and it's probably a cheaper option to use metformin than insulin.

Pregnancies to be cautious with metformin is if babies are likely to be small-for-dates [small-for-gestational-age] babies, so pregnancies in which there's pre-eclampsia that might slow the growth of the babies. Pregnancies where there's evidence of placental insufficiency, a slow growing baby, metformin is probably best avoided in those situations.

And a reminder to us, all the other diabetic agents are SGLT2s [sodium-glucose co-transporter 2 inhibitors], GLP-1s [glucagon-like peptide-1 receptor agonists], DPP-4s [dipeptidyl peptidase-4 inhibitors] and sulfonylureas not recommended.

Yeah, all of those are not recommended.

Chris, thanks for going through the detailed care for gestational diabetes. And so, to wrap up, let's go back to my territory. So, we're thinking about postpartum follow-up of gestational diabetes, which mostly is going to be in general practice. What are our follow-up points?

Traditionally, we've gone for a repeat glucose tolerance test at 6 to 12 weeks after the baby's born. Six weeks can be quite difficult for a mum with a really small baby, so 12 weeks might be more practical for mums. I don't think there's a reason to want to do it before 12 weeks. Some places around the world are favouring a haemoglobin A1c at 12 weeks, and I know that New Zealand are favouring that because the glucose tolerance test is not a really popular test with mums. The thing about a haemoglobin A1c at 3 months with pregnancy is there's blood loss, there's changes in rates of red cell formation, et cetera, et cetera. Has it all settled down by 3 months postpartum and gone back to a steady state?

We might nearly be there, but maybe not quite. So, that's one reason why the glucose tolerance test has been favoured to do then. Glucose tolerance test is probably more sensitive of picking up mild abnormalities of impaired fasting glucose, impaired glucose tolerance, whereas the haemoglobin A1c at the lower ranges is not very sensitive at picking those with risk of developing diabetes. So, that's another reason why the glucose tolerance test might be better, but certainly a haemoglobin A1c is better than not doing any follow-up. Then it really depends on the relative risk of the individual woman, how often you would check again screening for diabetes development.

A lower risk woman, you might do it every 2 to 3 years. Higher risk women, particularly if they're from a high-risk ethnic group or they had required insulin or medical therapy during pregnancy, they'll be at higher risk, and you probably should be screening them each year. The other side of it is what the other metabolic risks and particular risks for cardiovascular disease might be, and screening for hypertension, screening for dyslipidaemia, making sure women are not smoking, et cetera and providing them opportunities for getting education about healthy lifestyle if that's required.

Lots of opportunities post pregnancy, although it's a time when the lives of mums are very busy and so it really is difficult, and to find ways to get the messages across and keeping that follow-up going can be challenging and general practice, that's where it needs to be led from.

We are well placed to do this for women and their families. That's all we have time for today. Thank you Chris, and to your colleagues for this great overview of the recent updates to gestational diabetes screening and diagnosis, as well as reminding us about the principles of management. I know I've refreshed my risk factors for gestational diabetes screening for my antenatal consults already.

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I'll remind our listeners that the full article is available for free on the Australian Prescriber website. If you're a GP, it is now even easier to record your CPD with Australian Prescriber. After reading the article, you can follow the link to the reflective questions and you'll have 1 hour of Educational Activities and 1 hour of Reviewing Performance uploaded for you. The views of the hosts and guests on this podcast are their own and may not represent Australian Prescriber or Therapeutic Guidelines. Christopher Nolan was a chief investigator on the Treatment of Booking Gestational Diabetes Mellitus study, which was funded by the National Health and Medical Research Council.

Christopher has received support from the Canberra Hospital Private Practice Fund to attend conferences related to diabetes in pregnancy and has contributed to the development of various guidelines on diabetes, including the Australian Evidence-based Clinical Guidelines for Diabetes. Christopher received an honorarium for his role as Diabetologia advisory board member from 2020 to 2022.