WASHINGTON — Continuous glucose monitoring (CGM) is widely used during pregnancy for individuals with type 1 diabetes — with pregnancy-specific target metrics now chosen and benefits on perinatal outcomes demonstrated — but more research is needed to elucidate its role in the growing population of pregnant people with type 2 diabetes and gestational diabetes (GDM). And overall, there are still “many more questions unanswered about CGM use in pregnancy than what we have answered,” Celeste Durnwald, MD, said at the biennial meeting of the Diabetes in Pregnancy Study Group of North America.
There’s much to learn about how to best interpret “the detailed and complex data that CGM provides,” and what targets in addition to time in range (TIR) are most important, said Dr. Durnwald, director of the perinatal diabetes program and associate professor of ob.gyn. at the Hospital of the University of Pennsylvania, Philadelphia, in a presentation on CGM.
Among other questions are whether fasting glucose is “as important in the era of CGM,” and whether there should be different glycemic targets for nocturnal versus daytime TIR, she said. Moreover, questions justifiably remain about whether the TIR targets for type 1 diabetes in pregnancy are indeed optimal, she said in a discussion period.
Ongoing research is looking at whether CGM can motivate and guide patients with GDM through diet and lifestyle changes such that “we can see changes in amounts of medication we use,” Dr. Durnwald noted in her presentation. “There’s a whole breadth of research looking at whether CGM can help predict diagnosis of GDM, large for gestational age, or preeclampsia, and what are the targets.”
Maternal hypoglycemia during pregnancy — a time when strict glycemic control is recommended to reduce the risk of congenital malformations and other fetal and neonatal morbidity — remains a concern in type 1 diabetes, even with widespread use of CGM in this population, said Barak Rosenn, MD, during a presentation on glycemic control in type 1 diabetes.
A pilot study of a newly designed pregnancy-specific closed-loop insulin delivery system, published last year (Diabetes Care. 2023;46:1425-31), has offered the first “really encouraging information about the ability to use our most up-to-date technology to help our type 1 patients maintain strict control and at the same time decrease their risk of severe hypoglycemia,” said Dr. Rosenn, a maternal-fetal medicine specialist at the Jersey City Medical Center, Jersey City, New Jersey.
Guidance for tight intrapartum glucose control, meanwhile, has been backed by little evidence, said Michal Fishel Bartal, MD, MS, and some recent studies and reviews have shown little to no effect of such tight control on neonatal hypoglycemia, which is the aim of the guidance.
“We need to reexamine current recommendations,” said Dr. Bartal, assistant professor in the division of maternal-fetal medicine at the University of Texas Health Science Center, Houston, during a presentation on intrapartum care. “There’s very limited evidence-based data for the way we manage people with diabetes [during labor and delivery].”
The Knowns And Unknowns of CGM in Pregnancy
The multicenter, international CONCEPTT trial (Continuous Glucose Monitoring in Pregnant Women With Type 1 Diabetes), published in 2017, was the first trial to demonstrate improvements in perinatal outcomes, and it “brought CGM to the forefront in terms of widespread use,” Dr. Durnwald said.
The trial randomized more than 300 patients with type 1 diabetes who were pregnant or planning pregnancy (both users of insulin pumps and users of multiple insulin injections) to continuous, real-time CGM in addition to finger-stick glucose monitoring, or standard finger-stick glucose tests alone. In addition to small improvements in A1c and 7% more TIR (without an increase in hypoglycemia), pregnant CGM users had reductions in large-for-gestational age (LGA) births (53% vs 69%, P = .0489), neonatal intensive care admissions lasting more than 24 hours, and severe neonatal hypoglycemia.
Numbers needed to treat to prevent adverse outcomes in the CONCEPTT trial were six for LGA, six for NICU admission, and eight for neonatal hypoglycemia.
Data from the CONCEPTT trial featured prominently in the development of consensus recommendations for CGM targets in pregnancy by an international expert panel endorsed by the American Diabetes Association. In its 2019 report, the group recommended a target range of 63-140 mg/dL for type 1 and type 2 diabetes during pregnancy (compared with 70-180 mg/dL outside of pregnancy), and a TIR > 70% for pregnant people with type 1 diabetes. (Targets for time below range and time above range are also defined for type 1.)
More data are needed, the group said, in order to recommend TIR targets for type 2 diabetes in pregnancy or GDM (Diabetes Care. 2019;42:1593-603). “Many argue,” Dr. Durnwald said, “that there could be more stringent targets for those at less risk for [maternal] hypoglycemia, especially our GDM population.”
There’s a question of whether even higher TIR would further improve perinatal outcomes, she said, “or will we reach a threshold where higher TIR doesn’t get us a [further] reduction in LGA or preeclampsia.”
And while TIR is “certainly our buzzword,” lower mean glucose levels have also been associated with a lower risk of LGA and other adverse neonatal outcomes. A 2019 retrospective study from Sweden, for instance, analyzed patterns of CGM data from 186 pregnant women with type 1 diabetes and found significant associations between elevated mean glucose levels (in the second and third trimesters) and both LGA and an adverse neonatal composite outcome (Diabetologia. 2019;62:1143-53).
Elevated TIR was also associated with LGA, but “mean glucose had the strongest association with the rate of LGA,” Dr. Durnwald said.
Similarly, a 2020 subanalysis of the CONCEPTT trial data found that a higher mean glucose at both 24 and 34 weeks of gestation was significantly associated with a greater risk of LGA (Diabetes Care. 2020;43:1178-84), and a smaller 2015 analysis of data from two randomized controlled trials of CGM in pregnant women with type 1 and type 2 diabetes found this association in trimesters 2 and 3 (Diabetes Care. 2015;38;1319-25).
The ADA’s Standards of Care in Diabetes (Diabetes Care. 2024;47:S282-S294) endorse CGM as an adjunctive tool in pregnancy — not as a replacement for all traditional blood glucose monitoring — and advise that the use of CGM-reported mean glucose is superior to the use of estimated A1c, glucose management indicator, and other calculations to estimate A1c. Changes occur in pregnancy, Dr. Durnwald pointed out. “Most experts will identify a [target] mean glucose < 120 mg/dL in those with type 1, but there’s potential to have a mean glucose closer to 100 in certainly our patients with GDM and some of our patients with type 2,” she said. To a lesser extent, researchers have also looked at the effect of CMG-reported glycemic variability on outcomes such as LGA, with at least two studies finding some association, and there has been some research on nocturnal glucose and LGA, Dr. Durnwald said. CGM “gives us the opportunity,” she said, “to think about nocturnal glucose as a possible target” for further optimizing diabetes management during pregnancy.