CGM Metrics and the Risk of Pregnancy Complications in T1D
Ina Geerts, Kaat Beunen, Liese Berger, Nancy Van Wilder, Dominique Ballaux, Gerd Vanhaverbeke, Youri Taes, Xavier-Philippe Aers, Frank Nobels, Liesbeth Van Huffel, Joke Marlier, Dahae Lee, Joke Cuypers, Vanessa Preumont, Sarah E. Siegelaar, Rebecca C. Painter, Annouschka Laenen, Pieter Gillard, Chantal Mathieu and Katrien Benhalima
CGM Metrics and the Risk of Pregnancy Complications in Type 1 Diabetes: A Secondary Exploratory Analysis of the CRISTAL Trial. iabetes Technol Ther. 2026 Aug 12:15209156261477154.
Maintaining tight glucose control during pregnancy is essential for women with type 1 diabetes (T1D), as maternal hyperglycemia is associated with adverse maternal and neonatal outcomes. Pregnancy-specific time in range (TIRp; 3.5–7.8 mmol/L or 63–140 mg/dL) is increasingly used to assess glucose management during pregnancy. However, less is known about the potential value of other continuous glucose monitoring (CGM) metrics, particularly overnight glucose levels and glycemic variability.
This secondary exploratory analysis of the CRISTAL trial investigated whether different CGM metrics were associated with pregnancy complications in 95 women with T1D. CGM data collected throughout pregnancy were used to assess overall and overnight TIRp, time above range (TARp), time below range (TBRp), mean glucose, glycemic variability and the glucose management indicator (GMI). The study also evaluated whether combining CGM metrics could improve identification of pregnancies at higher risk of adverse outcomes and examined the relationship between GMI and HbA1c during pregnancy.
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Key findings:
- Higher TIRp was associated with a lower risk of several pregnancy complications: Every 5% increase in overall TIRp was associated with 37% lower odds of gestational hypertension, 44% lower odds of birthweight >4.5 kg, and substantially lower odds of neonatal hypoglycemia requiring hospital care.
- Overnight glucose control may provide additional information: Every 5% increase in overnight TIRp was associated with 29% lower odds of gestational hypertension and lower odds of neonatal hypoglycemia requiring hospital care. Conversely, higher overnight TARp was associated with increased odds of hospital care for neonatal hypoglycemia.
- More time above range was associated with adverse neonatal outcomes: Every 5% increase in overall TARp was associated with higher odds of birthweight >4.5 kg, respiratory distress, and neonatal hypoglycemia requiring hospital care.
- Glycemic variability also appeared relevant: Higher glucose variability, measured by standard deviation (SD), was associated with higher odds of gestational hypertension and birthweight >4.5 kg.
- Combining CGM metrics may provide more information than using a single metric: Combinations including TIRp or TARp, overnight TIRp/TARp, mean glucose and glycemic variability generally showed better discriminative performance for several pregnancy complications than individual CGM metrics.
- GMI and HbA1c were not interchangeable during pregnancy: Agreement between CGM-derived GMI and laboratory-measured HbA1c varied throughout pregnancy and was particularly discordant in the second trimester, when GMI systematically overestimated HbA1c. The findings support cautious interpretation of GMI as a substitute for HbA1c during pregnancy.
These findings suggest that evaluating CGM data beyond overall TIRp may provide additional information about the risk of pregnancy complications in women with T1D. In particular, overnight glucose control and glycemic variability could complement established CGM targets, while combining multiple CGM metrics may offer a more complete picture of glucose exposure during pregnancy.
However, this was an exploratory secondary analysis with a relatively small sample size, and the CRISTAL trial was not powered to assess pregnancy outcomes. The findings therefore require confirmation in larger studies before these CGM metrics can be used to predict individual pregnancy outcomes or to define separate overnight treatment targets.
Concluding, the authors state
"Specific combinations of CGM metrics, including overnight TIRp/TARp, may be informative for predicting pregnancy outcomes. GMI and HbA1c should not be considered interchangeable for glycemic control during pregnancy."
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