# Continuous Glucose Monitoring in Pregnant Women with Type 1 Diabetes: A Comprehensive Synthesis of the CONCEPTT Trial and Supporting Evidence on Large-for-Gestational-Age Outcomes

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### Background and Context

Pregnancy complicated by pre-existing type 1 diabetes mellitus (T1DM) remains one of the most challenging clinical scenarios in modern obstetrics and diabetology. Despite decades of advances in insulin therapy, glucose monitoring technology, and multidisciplinary antenatal care, women with T1DM continue to experience substantially elevated rates of adverse maternal and neonatal outcomes compared with the general obstetric population. Among the most clinically significant and persistent of these complications is the delivery of a large-for-gestational-age (LGA) infant — defined as a birthweight above the 90th percentile for gestational age, sex, and other relevant maternal characteristics. LGA infants born to mothers with T1DM face elevated risks of neonatal hypoglycaemia, hyperbilirubinaemia, respiratory distress syndrome, shoulder dystocia, birth injury, and admission to the neonatal intensive care unit (NICU). In the longer term, intrauterine exposure to maternal hyperglycaemia and the resulting fetal hyperinsulinaemia are associated with a 1.5-fold higher risk of childhood obesity and a 7.4-fold higher risk of type 2 diabetes mellitus in the offspring (PMC11928751). These intergenerational consequences make the prevention of LGA not merely a perinatal concern but a public health imperative with implications extending across the life course.

The pathophysiological basis for LGA in T1DM pregnancies is well established in broad outline: maternal hyperglycaemia drives transplacental glucose transfer, stimulating fetal pancreatic insulin secretion and promoting anabolic fetal growth. Nationwide UK audit data cited in the CONCEPTT trial protocol and results papers confirm that only 15% of pregnant women with T1DM achieve target HbA1c levels during early pregnancy, and even with intensive support — including fortnightly antenatal clinic visits and frequent between-clinic contacts — only 40% achieve target HbA1c levels after 24 weeks' gestation (PMC5713979). Despite this intensive clinical infrastructure, one in two newborn infants of mothers with T1DM experiences complications associated with maternal hyperglycaemia exposure, and there has been no meaningful improvement in neonatal morbidity over the past three to four decades (PMC5713979). This stagnation in outcomes, despite improvements in insulin formulations and delivery systems, underscores the inadequacy of conventional self-monitoring of blood glucose (SMBG) — typically performed seven times daily via fingerstick capillary testing — as the sole tool for glycaemic management in this high-risk population.

Against this backdrop, real-time continuous glucose monitoring (RT-CGM) emerged as a potentially transformative technology. CGM systems use subcutaneous interstitial glucose sensors to provide near-continuous glucose readings — up to 288 data points per day — along with trend arrows indicating the direction and rate of glucose change. Unlike SMBG, which captures only discrete point-in-time glucose values, CGM reveals the full glycaemic profile including postprandial excursions, nocturnal hypoglycaemia, and the duration and magnitude of hyperglycaemic episodes that are invisible to conventional monitoring. In non-pregnant adults with T1DM, randomised trials had demonstrated that regular CGM use (defined as at least 6 days per week) was associated with modest but clinically meaningful improvements in HbA1c — a mean reduction of approximately 0.3%, with reductions of up to 1.0% in those with poor baseline glycaemic control who used CGM consistently (PMC4950103). However, prior to the CONCEPTT trial, evidence in pregnant women was limited, conflicting, and derived from studies using older, less accurate CGM technology or intermittent rather than continuous monitoring protocols. The critical question — whether RT-CGM used continuously from early pregnancy could reduce not only maternal HbA1c but also the incidence of LGA and other clinically important neonatal outcomes — remained unanswered. The CONCEPTT trial was designed and powered to address this question directly, and its 2017 publication in The Lancet represents the landmark evidence base for the user's research question (PMC5713979).

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### Molecular and Mechanistic Pathways

#### Maternal Hyperglycaemia, Fetal Hyperinsulinaemia, and the Pedersen Hypothesis

The foundational mechanistic framework linking maternal glycaemia to fetal overgrowth is the Pedersen hypothesis, which posits that maternal hyperglycaemia drives transplacental glucose transfer, stimulating fetal pancreatic beta-cell hyperplasia and hyperinsulinaemia, which in turn promotes anabolic fetal growth and adiposity. This model is well supported by the HAPO (Hyperglycaemia and Adverse Pregnancy Outcomes) study, which demonstrated a continuous, graded, positive association between maternal glucose levels — even below the diagnostic threshold for gestational diabetes — and adverse pregnancy outcomes including LGA, macrosomia, and neonatal hypoglycaemia (PMC5852427). In the context of T1DM, where maternal glucose excursions are typically larger in magnitude and more unpredictable than in gestational diabetes, the fetal exposure to hyperglycaemia is correspondingly more severe and more variable.

The CONCEPTT trial provides direct mechanistic evidence consistent with the Pedersen hypothesis. Women randomised to CGM spent 68% of their time within the recommended glucose control target range of 3.5–7.8 mmol/L, compared with 61% for control participants — a difference of 7 percentage points, translating to approximately 1.7 additional hours per day in target. CGM users also spent 27% of their time above 7.8 mmol/L compared with 32% for controls, equivalent to approximately 1 hour less per day spent hyperglycaemic (PMC5713979). These direct CGM-derived measures of day-to-day exposure to maternal hyperglycaemia are mechanistically more relevant to fetal outcomes than surrogate markers such as HbA1c, which integrates glucose over 8–12 weeks and is further confounded during pregnancy by gestational changes in red cell turnover, anaemia, and iron supplementation (PMC5713979). The reduction in LGA observed in the CGM group — from 69% to 53% — is therefore most plausibly attributed to the cumulative reduction in fetal glucose exposure achieved through more responsive, real-time insulin dose adjustment enabled by CGM data.

Cord blood C-peptide, a marker of fetal hyperinsulinaemia, was measured in the CONCEPTT trial as a prespecified secondary outcome. In the CONCEPTT trial, cord blood C-peptide was measured as a prespecified secondary outcome but no statistically significant between-group difference in cord blood C-peptide levels was reported (PMC5713979). The directional consistency of the neonatal outcome improvements — reduced LGA, reduced neonatal hypoglycaemia, reduced NICU admission — is mechanistically coherent with reduced fetal hyperinsulinaemia secondary to reduced maternal hyperglycaemia exposure. The secondary analysis of CONCEPTT biospecimens examining circulating extracellular vesicles (EVs) provides additional mechanistic insight: higher baseline endothelial EV levels were independently associated with NICU admission, composite neonatal outcome, and hyperbilirubinaemia after adjustment for HbA1c and blood pressure (PMC8419913). This suggests that maternal vascular health, beyond glycaemia per se, may contribute to neonatal risk in T1DM pregnancies.

#### Glycaemic Variability as a Distinct Mechanistic Driver

A growing body of evidence, synthesised in multiple papers in this review, suggests that glycaemic variability — the magnitude and frequency of glucose excursions above and below target — contributes to adverse fetal outcomes independently of mean glucose levels or HbA1c. The mean amplitude of glycaemic excursion (MAGE), a CGM-derived metric of intraday glucose variability, has been identified as an independent risk factor for macrosomia (OR 1.90; 95% CI 1.19–3.04), preeclampsia (OR 3.66; 95% CI 2.16–6.20), and neonatal hypoglycaemia (OR 1.63; 95% CI 1.07–2.48) in women with gestational diabetes (PMC12470761). In the CONCEPTT trial, CGM users demonstrated significantly reduced glucose standard deviation at 34 weeks' gestation compared with controls (p=0.0359) and significantly reduced MAGE (p=0.0455), suggesting that the glycaemic variability reduction achieved by CGM may be a mechanistically important contributor to the observed neonatal outcome improvements, beyond the modest HbA1c reduction alone (PMC5713979).

The mechanistic basis for variability-driven fetal harm involves multiple pathways. Glucose fluctuations trigger endothelial dysfunction through activation of oxidative stress pathways, including reactive oxygen species (ROS) generation, which damages endothelial cells and promotes a pro-inflammatory, pro-coagulant vascular phenotype (PMC5852427). This is consistent with the CONCEPTT EV secondary analysis finding that endothelial EV levels — markers of endothelial injury — showed a weak but statistically significant positive correlation with MAGE (r=0.129, 95% CI 0.005–0.248, p=0.04), though not with mean glucose or HbA1c (PMC8419913). Chronic low-grade inflammation driven by adipose tissue macrophage-derived TNF-α and IL-6 activates stress kinases including JNK and IKK, promoting serine phosphorylation of IRS-1 and exacerbating systemic insulin resistance, which may further destabilise glycaemic control in maternal tissues (PMC12429770). In the placenta, glucose-induced oxidative stress and mitochondrial dysfunction impair trophoblast function and may alter placental permeability to glucose and other nutrients, amplifying fetal exposure to metabolic perturbations beyond what maternal blood glucose levels alone would predict (PMC12429770).

A particularly important mechanistic observation from CGM studies in gestational diabetes is that nocturnal hyperglycaemia — largely invisible to conventional SMBG — is independently associated with increased birth weight percentile. Each standard deviation increase in nighttime mean glucose level was associated with a 6.0-point increase in birth weight percentile (95% CI 0.4–11.5), and each standard deviation increase in hours per day spent in severe glycaemic variability was associated with a 6.3-point increase in birth weight percentile (95% CI 0.4–12.2) (PMC12470761). CGM's unique ability to capture nocturnal glucose profiles — including the detection of nocturnal hyperglycaemia that would otherwise go undetected and untreated — is therefore a mechanistically plausible explanation for part of the LGA reduction observed in the CONCEPTT trial, where CGM users showed improved overnight as well as daytime glycaemic control (PMC5713979).

#### Non-Glycaemic Contributors to LGA in T1DM Pregnancies

A critical mechanistic insight from the supporting literature is that glucose is not the sole determinant of fetal overgrowth in T1DM pregnancies, and that the persistent high rates of LGA even in well-controlled T1DM populations reflect the contribution of non-glycaemic factors. A Belgian retrospective cohort study of 259 T1DM pregnancies over 22 years found that excessive gestational weight gain (OR 1.95, 95% CI 1.08–3.53, p=0.027) was an independent predictor of LGA in multivariable analysis, alongside first-trimester HbA1c (OR 1.43, 95% CI 1.05–1.95, p=0.023) and third-trimester HbA1c (OR 1.70, 95% CI 1.07–2.71, p=0.026), while pre-pregnancy BMI was not independently significant (PMC4946226). Notably, the proportion of overweight women in this cohort nearly doubled over the 22-year study period (from 23.3% to 39.3%, p=0.009), and excessive weight gain increased from 21.3% to 37.7% (p=0.019), providing a plausible explanation for why LGA rates remained persistently high (45.2% overall, ranging from 35.9% to 53.2% across time epochs) despite generally good glycaemic control (PMC4946226).

Maternal serum triglycerides demonstrate the strongest predictive relationship with fetal growth in some analyses, and adiponectin — which typically decreases from mid-pregnancy, most pronouncedly in mothers delivering LGA infants — may independently contribute to LGA through effects on placental nutrient transport (PMC5852427, PMC12429770). These non-glycaemic pathways explain why the CONCEPTT trial authors themselves noted that "even with the improvement obtained with CGM use, pregnancy outcomes remained suboptimal, with a high proportion of infants large for gestational age and high levels of neonatal morbidity," and that "in addition to glucose, other nutrients might also be important" (PMC5713979). The CONCEPTT trial authors further observed, within the trial's own participant data, that the LGA rate in the control group (69%) substantially exceeded that reported in some national audit populations, and that even the CGM group's rate (53%) remained far above the expected population baseline — a pattern that underscores the importance of non-glycaemic determinants of fetal growth that persist regardless of monitoring modality (PMC5713979).

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### Dose-Response and Pharmacokinetics

#### CGM Sensor Compliance and Dose-Response Relationship

In the context of CGM as an intervention, the relevant "dose" is the frequency and consistency of sensor use, which determines the completeness of glucose data available to the user and clinician for real-time insulin dose adjustment. The median frequency of CGM use in the CONCEPTT pregnancy trial was 6.1 days per week (IQR 4.0–6.8), with 70% of pregnant participants using CGM for more than 75% of the time; sensor use was highest at 25–34 weeks (median 6.5 days per week, IQR 3.9–7.0) (PMC5713979). Sensor compliance was generally high, which is mechanistically important because the second and third trimesters are the periods of most rapid fetal growth and greatest fetal sensitivity to maternal hyperglycaemia.

Prior evidence from non-pregnant populations had established that the glycaemic benefit of CGM is dose-dependent: regular use defined as at least 6 days per week was associated with maximal HbA1c reductions of up to 1.0% in those with poor baseline glycaemic control (PMC4950103). The CONCEPTT trial's achievement of median use approaching 6 days per week, with 70% of participants using CGM more than 75% of the time, likely contributed to the trial's ability to detect significant improvements in both glycaemic metrics and neonatal outcomes — in contrast to the earlier Danish trial, which used RT-CGM intermittently (six days × five occasions) and found no difference in glycaemic control or neonatal outcomes, possibly because only 60% of women used it intermittently (PMC4950103, PMC5713979).

#### Glycaemic Target Ranges and Time-in-Range Thresholds

According to the International Consensus on Time in Range, pregnancy-specific CGM targets have been established: time in range (TIR) greater than 70% with a glycaemic target range of 63–140 mg/dL (3.5–7.8 mmol/L); level 1 time below range (TBR) less than 4% with the glycaemic target below 63 mg/dL; level 2 TBR less than 1% of readings below 54 mg/dL; and time above range (TAR) less than 25% of readings above 140 mg/dL (PMC12470761). In the CONCEPTT trial, CGM users achieved 68% TIR at 34 weeks' gestation compared with 61% for controls (p=0.0034), and 27% TAR compared with 32% for controls (p=0.0279) — both statistically significant improvements, though neither group fully achieved the consensus TIR target of greater than 70% (PMC5713979). This observation is clinically important: it suggests that even partial improvements in TIR — in this case, a 7 percentage point improvement translating to approximately 1.7 additional hours per day in target — are sufficient to produce clinically meaningful reductions in LGA and other neonatal outcomes.

The relationship between TIR improvements and LGA reduction has been further quantified in the gestational diabetes literature. In women with T1DM, a 5–7% increase in TIR in the second half of pregnancy has been associated with a lower risk of LGA and improved neonatal outcomes including lower rates of macrosomia, neonatal hypoglycaemia, and NICU admissions (PMC12470761). In GDM populations, each percentage point increase in TAR was associated with a 24% increase in the probability of pharmacological treatment requirement (OR 1.24; 95% CI 1.03–1.45, p=0.006), and TAR ≥5% was linked to an increased risk of premature delivery (OR 2.17, 95% CI 1.11–4.24) (PMC12470761). These quantitative relationships provide a mechanistic dose-response framework for understanding why the modest but consistent glycaemic improvements achieved by CGM in the CONCEPTT trial translated into clinically significant neonatal outcome improvements.

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### Clinical Evidence and Implications

#### The CONCEPTT Trial: Design and Primary Glycaemic Outcomes

The CONCEPTT (Continuous Glucose Monitoring in Pregnant Women with Type 1 Diabetes) trial is the definitive randomised controlled trial addressing the user's research question. The CONCEPTT trial was a multicentre, international, open-label RCT conducted across 31 hospitals in Canada, England, Scotland, Spain, Italy, Ireland, and the USA, enrolling participants between March 2013 and March 2016, comprising a pregnancy trial (n=215; 108 CGM, 107 control) and a planning pregnancy trial (n=110; 53 CGM, 57 control) (PMC5713979). Eligible participants were women aged 18–40 years with T1DM of at least 12 months' duration, receiving intensive insulin therapy via multiple daily injections or insulin pump, with HbA1c between 6.5–10.0% (48–86 mmol/mol) for the pregnancy cohort and 7.0–10.0% (53–86 mmol/mol) for the planning pregnancy cohort. Pregnant women were eligible if they had a live singleton fetus confirmed by ultrasound at 13 weeks and 6 days' gestation or less (PMC5713979, PMC4950103).

The primary outcome was the difference in change in HbA1c from randomisation to 34 weeks' gestation in the pregnancy trial. In the CONCEPTT pregnancy trial, CGM produced a statistically significant reduction in HbA1c compared with controls: mean difference −0.19% (95% CI −0.34 to −0.03; p=0.0207), with mean HbA1c at 34 weeks of 6.35% in the CGM group versus 6.53% in the control group (PMC5713979). The proportion of women achieving the target HbA1c of ≤6.5% at 34 weeks was 66% (63/95) in the CGM group versus 52% (48/92) in the control group (p=0.0601, not statistically significant). In the CONCEPTT planning pregnancy trial, the between-group HbA1c difference was −0.17% (95% CI −0.43 to 0.09; p=0.20) — not statistically significant, likely reflecting the smaller sample size (PMC5713979).

The HbA1c improvement, while statistically significant, was clinically modest — a 0.19 percentage point difference. The CONCEPTT investigators themselves acknowledged that "the clinically small improvement in HbA1c seems unlikely to be the sole explanation for the observed reductions in neonatal complications," and that direct CGM measures of day-to-day glucose exposure — particularly time in target and time above target — are likely more mechanistically relevant to neonatal outcomes than HbA1c (PMC5713979). This observation has important implications for how CGM's clinical benefit should be conceptualised: not primarily as an HbA1c-lowering intervention, but as a tool that reduces the cumulative burden of hyperglycaemic exposure in ways that HbA1c cannot fully capture.

#### The CONCEPTT Trial: Large-for-Gestational-Age as the Primary Neonatal Outcome

The most clinically impactful finding of the CONCEPTT trial, directly answering the user's research question, is the statistically significant reduction in LGA in infants of mothers randomised to CGM. LGA (birthweight >90th centile) occurred in 53% of infants in the CONCEPTT CGM group versus 69% in the control group — an absolute risk reduction of 16 percentage points, OR 0.51 (95% CI 0.28–0.90; p=0.0210), with a number needed to treat of six (PMC5713979). The number needed to treat (NNT) to prevent one LGA infant was six — a clinically meaningful and practically achievable treatment effect.

This finding is robust across several sensitivity analyses. No heterogeneity in the CGM treatment effect on LGA was detected across the seven participating countries in CONCEPTT (likelihood ratio for heterogeneity 0.14; p=0.99), indicating that the benefit was consistent regardless of the clinical setting or background population LGA rates (PMC5713979). The treatment effect was also comparable between women using insulin pumps and those using multiple daily injections, which is particularly important for widening access to CGM technology — demonstrating that the benefit is not restricted to the most technologically sophisticated insulin delivery method (PMC5713979).

The pre-specified secondary analysis examining which growth standard best identifies LGA in T1DM pregnancies confirmed the CONCEPTT finding using multiple methodologies. In the CONCEPTT growth standards secondary analysis of 225 mother-infant pairs, LGA rates were 62.2% by GROW, 66.7% by INTERGROWTH-21st, and 29.8% by WHO standards; the substantially lower WHO LGA rate reflects the WHO standard's failure to incorporate gestational age at birth — a critical limitation given the preterm delivery rate of 39.6% in this cohort (PMC7845036). GROW-defined LGA (>90th centile) was significantly associated with neonatal hypoglycaemia (RR 1.86, 95% CI 1.09–3.20), NICU admission (RR 1.49, 95% CI 1.01–2.21), and composite neonatal outcome (RR 1.56, 95% CI 1.13–2.16); infants above the 97.7th centile by GROW showed stronger associations including neonatal hypoglycaemia (RR 2.35, 95% CI 1.47–3.75) and NICU admission (RR 1.62, 95% CI 1.15–2.28) (PMC7845036).

#### The CONCEPTT Trial: Additional Neonatal Outcomes

Beyond LGA, the CONCEPTT trial demonstrated statistically significant improvements in three additional clinically important neonatal outcomes in the CGM group. In the CONCEPTT trial, NICU admission requiring at least 24 hours occurred in 27% of infants in the CGM group versus 43% in the control group (OR 0.48, 95% CI 0.26–0.86; p=0.0157; NNT=6), and neonatal hypoglycaemia requiring intravenous dextrose occurred in 15% versus 28% (OR 0.45, 95% CI 0.22–0.89; p=0.0250; NNT=8) (PMC5713979). Total infant length of hospital stay was significantly shorter in the CONCEPTT CGM group (p=0.0091), with median stays of 3.1 days (IQR 2.1–5.7) versus 4.0 days (IQR 2.4–7.0) in the control group — a one-day reduction that has substantial implications for healthcare resource utilisation and family wellbeing (PMC5713979).

The composite neonatal outcome in CONCEPTT occurred in 42.9% of infants in the CGM group versus 52.8% in the control group (p=0.17). Hyperbilirubinaemia occurred in 25% versus 31% (p=0.43) and respiratory distress in 9% versus 9% (p=1.0) — none reaching statistical significance (PMC5713979). This non-significant composite result, despite significant improvements in individual components, likely reflects the heterogeneity of the composite endpoint and the relatively modest sample size for detecting differences in a composite that includes rare events such as pregnancy loss and birth injury.

#### The CONCEPTT Trial: Maternal Outcomes

In contrast to the significant improvements in neonatal outcomes, the CONCEPTT trial found no statistically significant differences between CGM and control groups in maternal obstetric outcomes. As reported in the CONCEPTT pregnancy trial, no statistically significant differences were found in maternal outcomes: hypertensive disorders (18% vs. 27%, p=0.13), pre-eclampsia (9% vs. 18%, p=0.10), caesarean section (63% vs. 73%, p=0.18), gestational weight gain (13.1 vs. 13.7 kg, p=0.22), preterm birth before 37 weeks (38% vs. 42%, p=0.57), or early preterm birth before 34 weeks (5% vs. 11%, p=0.19) (PMC5713979). The directional trends for pre-eclampsia and preterm birth favoured CGM, and the pre-eclampsia comparison approached but did not reach statistical significance (p=0.10), suggesting that a larger trial might have detected a significant maternal benefit. The trial was powered primarily for the HbA1c primary outcome and was not specifically powered to detect differences in individual maternal outcomes, which are less common than neonatal outcomes in this population.

#### Supporting Evidence from Observational Studies and Retrospective Cohorts

The CONCEPTT trial's findings on LGA are contextualised and supported by a substantial body of observational evidence linking glycaemic control quality to fetal overgrowth in T1DM pregnancies. The Belgian retrospective cohort study of 259 T1DM pregnancies over 22 years (1992–2014) found an overall LGA rate of 45.2%, with no significant improvement over time despite generally good glycaemic control (PMC4946226). In multivariable logistic regression, first-trimester HbA1c (OR 1.43, 95% CI 1.05–1.95, p=0.023) and third-trimester HbA1c (OR 1.70, 95% CI 1.07–2.71, p=0.026) were independent predictors of LGA, confirming that glycaemic control throughout pregnancy — not just at a single time point — influences fetal growth outcomes (PMC4946226). In the Belgian retrospective cohort of 259 T1DM pregnancies, women with HbA1c below 42 mmol/mol (6.0%) in the first trimester had significantly lower odds of LGA (OR 0.47, 95% CI 0.26–0.86, p=0.015), and those with HbA1c below 42 mmol/mol in the second trimester had similarly reduced odds (OR 0.47, 95% CI 0.27–0.80, p=0.005) (PMC4946226).

The Chinese retrospective cohort study of 2,860 diabetes-in-pregnancy cases (2018–2019) provides complementary evidence from a different clinical context. In a Chinese retrospective cohort of 2,860 diabetes-in-pregnancy cases (2018–2019), poor glycaemic control was associated with an adjusted OR of 2.479 (95% CI 1.606–3.825) for LGA, and pregestational diabetes mellitus (versus gestational diabetes) was associated with an adjusted OR of 1.845 (95% CI 1.051–3.240) for LGA; the overall LGA incidence was 3.95% (PMC9118638). These findings confirm that the quality of glycaemic control — not merely the presence of diabetes — is the critical determinant of LGA risk, and that pre-existing diabetes (including T1DM) confers higher LGA risk than gestational diabetes even after adjustment for glycaemic control quality.

A systematic review and meta-analysis of HbA1c and LGA in gestational diabetes (17,711 women across 23 observational studies) found a pooled relative risk for LGA of 1.70 (95% CI 1.23–2.35, p=0.001) in women with high versus normal/low HbA1c at 24–28 weeks, with substantial between-study heterogeneity (I²=68.3%, p=0.008) (PMC10253627). While this meta-analysis focused on gestational diabetes rather than T1DM, the mechanistic pathway — maternal hyperglycaemia driving fetal overgrowth — is shared, and the quantitative relationship between HbA1c elevation and LGA risk is directionally consistent with the CONCEPTT trial findings.

#### Evidence from CGM Studies in Gestational Diabetes: Contextualising the CONCEPTT Findings

Multiple systematic reviews and RCTs examining CGM in gestational diabetes provide important contextual evidence for interpreting the CONCEPTT findings, though they address a different patient population. The 2025 systematic review of CGM in GDM (PROSPERO CRD42024559169) synthesised evidence from multiple RCTs and clinical trials, finding that CGM use was associated with improved glycaemic control, reduced gestational weight gain, lower hypoglycaemia rates, and improved neonatal outcomes in several studies (PMC12470761). The 2025 systematic review of CGM in GDM found that one larger study reported statistically significant reductions in neonatal hypoglycaemia (5.5% vs. 14%, p=0.011), hyperbilirubinaemia (2.7% vs. 9.7%, p=0.012), respiratory distress syndrome (1.4% vs. 5.9%, p=0.034), and composite neonatal outcome (27.4% vs. 49.5%, p<0.001) in the CGM group; another study found fewer LGA fetuses (20 vs. 48, p=0.025) and lower macrosomia rate (4.1% vs. 10.8%, p=0.01) in the CGM group (PMC12470761).

However, the 2019 systematic review of CGM in pregnancy (29 studies, 1998–2018) found more mixed results. That review found that two RCTs and one prospective cohort showed no differences in any maternal or fetal outcomes between CGM and SMBG groups in GDM, while three other studies — including one RCT and two prospective cohorts — reported otherwise (PMC6798167). Among fetal outcomes specifically, all three RCTs in that review found no significant differences between CGM and SMBG groups, and only one study (Yu et al., n=340) reported significantly better fetal outcomes in the CGM group, including lower rates of macrosomia, LGA, neonatal hypoglycaemia, hyperbilirubinaemia, and respiratory distress syndrome (PMC6798167). The authors of this review concluded that "most studies showed inconclusive results regarding CGM use in order to improve maternal and fetal outcomes," attributing the null findings primarily to inadequate statistical power in small studies (PMC6798167). This heterogeneity in the GDM literature underscores the importance of the CONCEPTT trial's larger sample size and continuous (rather than intermittent) CGM use protocol in achieving definitive results in the T1DM population.

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### Safety Profile and Adverse Events

#### Hypoglycaemia Risk with CGM Use

A primary safety concern with CGM use in pregnancy is the potential for increased hypoglycaemia, either through more aggressive insulin dosing enabled by real-time glucose data or through alarm fatigue leading to inappropriate responses to low glucose alerts. The CONCEPTT trial found no significant difference in severe hypoglycaemia rates between CGM and control groups. In the CONCEPTT trial, severe hypoglycaemia (requiring third-party assistance) occurred in 11% of CGM participants versus 12% of control participants (p=1.0), and time spent below target (<3.5 mmol/L) was 3% in the CGM group versus 4% in the control group (p=0.10) (PMC5713979). These findings are reassuring: the glycaemic improvements achieved by CGM were not purchased at the cost of increased hypoglycaemia risk.

The CONCEPTT trial authors noted that the burden of severe hypoglycaemia in their study (11% of 207 women with 39 episodes) was substantially lower than in the earlier Danish trial (16% of 119 women with 59 episodes), suggesting that over the past decade, advances in CGM technology and insulin management have substantially reduced the burden of severe hypoglycaemia in T1DM pregnancies (PMC5713979). CGM users in CONCEPTT showed a significant decrease in hypoglycaemia avoidance behaviours over time (HFS Behavior subscale, p=0.0347), suggesting that real-time glucose data may reduce the fear-driven behaviours that paradoxically increase hypoglycaemia risk by causing women to maintain higher glucose levels as a safety buffer (PMC5713979).

#### Device-Related Adverse Events

The most common adverse events in the CONCEPTT trial were device-related. Skin reactions at the sensor insertion site occurred in 49 (48%) of 103 CGM participants during the CONCEPTT pregnancy trial, compared with 8 (8%) of 104 control participants; more than 80% of CGM users reported frustrations with CGM use including connectivity issues, alarms, and calibration errors (PMC5713979). These device-related adverse events, while not medically serious, represent a significant burden on users and may affect adherence and quality of life. The overall rate of adverse events was higher in the CGM group (48% vs. 40% of participants with any adverse event; OR 1.2, 95% CI 0.8–1.8; p=0.35), though this difference was not statistically significant (PMC5713979). Serious adverse events were rare and comparable between groups in CONCEPTT (8 in the CGM group vs. 5 in the control group; OR 1.6, 95% CI 0.5–4.9; p=0.41), with no maternal deaths and no CGM-attributable serious adverse events (PMC5713979).

#### Unscheduled Clinical Contacts and Healthcare Resource Implications

An unexpected finding in the CONCEPTT trial was the substantially higher number of unscheduled clinical contacts in the CGM group compared with controls. In the CONCEPTT trial, CGM participants made substantially more unscheduled clinical contacts than controls (1,530 vs. 1,026 during pregnancy), attributed to sensor issues (mean 2.1 vs. 0.1 per participant; p<0.0001) and sensor-related diabetes management issues (2.6 vs. 0.2 per participant; p<0.0001); a post-hoc analysis confirmed this had minimal impact on the CGM treatment effect estimate (p=0.0427) (PMC5713979). This finding suggests that the increased clinical contact associated with CGM use does not explain the glycaemic or neonatal outcome improvements, but it does have implications for healthcare resource planning and the cost-effectiveness of CGM implementation in antenatal care.

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### Unresolved Controversies and Disagreements

#### The Magnitude of HbA1c Improvement and Its Relationship to Neonatal Outcomes

A central unresolved question raised by the CONCEPTT trial is the mechanistic relationship between the modest HbA1c improvement achieved by CGM (−0.19%, 95% CI −0.34 to −0.03) and the clinically substantial neonatal outcome improvements observed. The CONCEPTT investigators themselves acknowledged this apparent discrepancy, noting that "the clinically small improvement in HbA1c seems unlikely to be the sole explanation for the observed reductions in neonatal complications" (PMC5713979). The CONCEPTT trial authors also noted that the UK pregnancy in diabetes audit, which used the same customised birthweight charts, reported a lower proportion of infants large for gestational age (46%) despite higher maternal HbA1c levels in early and late pregnancy and fewer women achieving target HbA1c levels (40% compared with 66% of pregnant CGM users) (PMC5713979). This observation — drawn from the CONCEPTT trial's own contextualisation of its findings — further underscores the importance of non-glycaemic determinants of fetal growth and the complexity of interpreting LGA rates across different clinical populations. This paradox suggests that CGM's benefit operates through mechanisms beyond HbA1c reduction, including reductions in glycaemic variability, postprandial hyperglycaemic excursions, and nocturnal hyperglycaemia — all of which are captured by CGM metrics but not by HbA1c.

The earlier UK trial of intermittent masked CGM (n=71 women with T1DM and T2DM) achieved a larger between-group HbA1c difference (0.6%) and found a reduction in macrosomia (OR 0.36, 95% CI 0.13–0.98) (PMC4950103). The earlier Danish trial of intermittent RT-CGM (n=154 women) found no difference in glycaemic control or neonatal outcomes, possibly because only 60% of women used CGM intermittently and the technology was an earlier generation (PMC4950103, PMC5713979). These conflicting results from earlier trials — one showing benefit with masked CGM, one showing no benefit with intermittent RT-CGM — highlight the importance of both CGM modality (masked vs. real-time) and frequency of use (intermittent vs. continuous) in determining clinical outcomes. The CONCEPTT trial's use of continuous RT-CGM from early pregnancy, with high sensor compliance, appears to be the critical design feature that enabled detection of significant neonatal outcome improvements.

#### Whether CGM Benefit Extends to the Pre-Pregnancy Period

The CONCEPTT trial found no significant benefit from CGM use in women planning pregnancy. The between-group HbA1c difference in the planning pregnancy trial was −0.17% (95% CI −0.43 to 0.09; p=0.20) — similar in magnitude to the pregnancy trial but not statistically significant (PMC5713979). The trial was underpowered for the planning pregnancy cohort (n=110 vs. n=215 in the pregnancy cohort), and the authors noted that the wider confidence interval could reflect either a smaller sample size or more variation in the treatment effect in the pre-pregnancy setting. The motivation of participants planning pregnancy for optimising glucose control may also have been more variable than the motivation of pregnant participants, with 50% of CGM participants and 40% of control group participants in the planning pregnancy trial achieving their glycaemic control target (PMC5713979). The obstetric and neonatal outcomes of the 34 women who conceived during the planning pregnancy trial were reported separately and their small numbers precluded further statistical analysis. Whether CGM use before conception — during the critical period of organogenesis — confers additional benefit beyond what is achieved by starting CGM in early pregnancy remains an important unanswered question.

#### Non-Glycaemic Determinants of LGA and the Limits of Glucose-Centric Management

A fundamental controversy in the field concerns the extent to which glucose-centric management strategies — including CGM — can normalise LGA rates in T1DM pregnancies, given the substantial contribution of non-glycaemic factors to fetal overgrowth. The CONCEPTT trial found that even in the CGM group, 53% of infants were LGA — a rate that remains far above the expected population rate of approximately 10% (PMC5713979). The Belgian cohort study found persistently high LGA rates (45.2%) over 22 years despite generally good glycaemic control, with increasing maternal overweight and excessive gestational weight gain identified as independent contributors (PMC4946226). The narrative review on GDM controversies synthesises evidence that maternal triglycerides, adiponectin, inflammatory mediators, gut microbiota, and maternal BMI all independently contribute to fetal overgrowth through pathways that are not addressed by glucose monitoring or insulin therapy alone (PMC5852427). This evidence collectively suggests that glucose-centric management, even when optimised through CGM, cannot fully normalise LGA rates in T1DM pregnancies, and that additional interventions targeting nutrition, weight management, lipid metabolism, and inflammation will be required to achieve further improvements.

#### Optimal CGM Metrics for Pregnancy Management

A related controversy concerns which CGM-derived metrics are most clinically relevant for guiding management and predicting outcomes in T1DM pregnancies. The International Consensus on Time in Range has endorsed TIR, TAR, and TBR targets for pregnancy, but these were derived primarily from T1DM data and their applicability to GDM and T2DM remains uncertain (PMC12470761). The CONCEPTT trial demonstrated significant improvements in TIR (68% vs. 61%, p=0.0034), TAR (27% vs. 32%, p=0.0279), glucose SD (p=0.0359), and MAGE (p=0.0455), but did not find a significant difference in the coefficient of variation (p=0.058) or time below target (p=0.10) (PMC5713979). The relative importance of these different metrics — and whether TIR, TAR, or variability measures are the most actionable targets for clinical management — remains an area of active investigation. The EV secondary analysis of CONCEPTT found that MAGE, but not mean glucose or HbA1c, correlated with endothelial EV levels (r=0.129, p=0.04), suggesting that glycaemic variability may be a particularly important mechanistic driver of vascular injury in T1DM pregnancies (PMC8419913).

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### Limitations of the Evidence Base

The CONCEPTT trial, while the largest and most methodologically rigorous RCT of CGM in T1DM pregnancy, has several important limitations that constrain the generalisability and interpretation of its findings. The CONCEPTT trial was open-label, did not collect data on the frequency of capillary glucose monitoring in the control group or on the use of insulin suspension features in pump users, and approximately 20% of HbA1c samples were missing due to withdrawals, lost samples, or participants being unavailable (PMC5713979).

The trial conducted a large number of secondary statistical tests without adjustment for multiple comparisons, which increases the risk of false-positive findings among the secondary outcomes; however, the primary neonatal outcomes (LGA, NICU admission, neonatal hypoglycaemia) were all prespecified and the consistency of directional effects across multiple outcomes reduces the likelihood that these represent chance findings (PMC5713979).

The CONCEPTT trial population was predominantly of European or Mediterranean origin (87% in the pregnancy trial), college-educated, and non-smoking, and participants had to demonstrate willingness and ability to use CGM during a run-in phase before randomisation, limiting generalisability (PMC5713979). The CGM device used (Medtronic Guardian REAL-Time or MiniMed Minilink system) was provided at discounted price by the manufacturer, and CGM technology has continued to evolve substantially since the trial was conducted — newer devices with improved accuracy, longer sensor wear times, and factory calibration may produce different results (PMC5713979).

The planning pregnancy trial was underpowered to detect differences of the magnitude seen in the pregnancy trial, with only 110 participants (PMC5713979, PMC4950103). This means that the null finding in the planning pregnancy cohort cannot be interpreted as evidence of no benefit — it may simply reflect insufficient statistical power. The trial was not powered to detect differences in individual rare outcomes such as stillbirth, congenital anomaly, or shoulder dystocia, and the composite neonatal outcome did not reach statistical significance despite significant improvements in individual components (PMC5713979).

The supporting observational literature has its own methodological limitations. The Belgian retrospective cohort study (PMC4946226) was conducted at a single tertiary centre with predominantly Caucasian patients and high insulin pump use rates (84.6%), limiting generalisability. The Chinese retrospective cohort study (PMC9118638) was conducted at a specialised referral centre for severely ill pregnant women, inflating the apparent incidence of adverse outcomes relative to the general population. The systematic reviews of CGM in GDM (PMC12470761, PMC6798167) included studies with heterogeneous designs, CGM devices, monitoring durations, GDM diagnostic criteria, and reported outcomes, precluding quantitative meta-analysis in most cases and limiting the strength of conclusions. The meta-analysis of HbA1c and LGA in GDM had substantial between-study heterogeneity (I²=68.3% for LGA, I²=75.2% for macrosomia) and no common HbA1c cut-off value across studies, limiting the precision of the pooled estimates (PMC10253627).

The CONCEPTT EV secondary analysis was limited to n=163 women with at least one plasma sample, restricted to three time points, and found no significant difference in EV levels between CGM and SMBG groups (PMC8419913).

The CONCEPTT growth standards secondary analysis found that the sensitivity and specificity of LGA classification for predicting individual neonatal outcomes were modest across all growth standards (sensitivity 71–77%, specificity 32–42% for LGA >90th centile predicting hypoglycaemia or NICU admission), and the cohort was 86.2% European/Mediterranean, limiting assessment of growth standard performance across ethnicities (PMC7845036).

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### Conclusions and Future Directions

#### What the Evidence Supports

The CONCEPTT trial provides definitive, high-quality randomised controlled trial evidence that real-time continuous glucose monitoring, used continuously from early pregnancy in women with pre-existing type 1 diabetes on intensive insulin therapy, significantly reduces the incidence of large-for-gestational-age infants compared with standard self-monitoring of blood glucose. The absolute risk reduction was 16 percentage points (53% vs. 69%; OR 0.51, 95% CI 0.28–0.90; p=0.0210), with a number needed to treat of six — a clinically meaningful effect size that compares favourably with many established obstetric interventions (PMC5713979). This LGA reduction was accompanied by significant reductions in NICU admission lasting more than 24 hours (27% vs. 43%; OR 0.48, 95% CI 0.26–0.86; p=0.0157; NNT=6), neonatal hypoglycaemia requiring intravenous dextrose (15% vs. 28%; OR 0.45, 95% CI 0.22–0.89; p=0.0250; NNT=8), and total infant length of hospital stay (p=0.0091) (PMC5713979). These findings were consistent across 31 international study sites, across women using insulin pumps and multiple daily injections, and across countries with different background LGA rates, supporting the generalisability of the benefit within the studied population.

The evidence supports the conclusion that CGM's benefit in reducing LGA operates through multiple mechanisms: a modest but significant reduction in HbA1c (−0.19%, p=0.0207), a clinically meaningful increase in time in target glucose range (68% vs. 61%, p=0.0034), a reduction in time above target (27% vs. 32%, p=0.0279), and reductions in glycaemic variability as measured by glucose SD (p=0.0359) and MAGE (p=0.0455) (PMC5713979). The evidence supports the mechanistic framework that cumulative reduction in fetal glucose exposure — achieved through more responsive, real-time insulin dose adjustment enabled by CGM data — is the primary driver of LGA reduction, consistent with the Pedersen hypothesis and the extensive observational literature linking maternal hyperglycaemia to fetal overgrowth (PMC5852427, PMC4946226, PMC10253627).

The evidence also supports the safety of CGM use in T1DM pregnancy: severe hypoglycaemia rates were not increased in the CGM group (11% vs. 12%, p=1.0), and time spent below target glucose range was not significantly different between groups (3% vs. 4%, p=0.10) (PMC5713979). The glycaemic improvements achieved by CGM were not purchased at the cost of increased hypoglycaemia risk — a critical safety consideration in pregnancy, where maternal hypoglycaemia can compromise fetal oxygen and nutrient delivery.

#### What the Evidence Does NOT Support

The evidence does not support the conclusion that CGM normalises LGA rates in T1DM pregnancies. Even in the CGM group, 53% of infants were LGA — a rate that remains far above the expected population rate of approximately 10% and above the rates reported in some national T1DM pregnancy audits (PMC5713979). This persistent high LGA rate, despite CGM use, reflects the contribution of non-glycaemic factors — including maternal overweight, excessive gestational weight gain, lipid abnormalities, adipokine dysregulation, and inflammatory pathways — that are not addressed by glucose monitoring or insulin therapy alone (PMC5852427, PMC4946226). The evidence does not support the use of CGM as a standalone intervention sufficient to normalise pregnancy outcomes in T1DM; it should be understood as one component of a comprehensive multidisciplinary management strategy.

The evidence does not support a significant benefit of CGM in the pre-pregnancy period, at least within the statistical power of the CONCEPTT planning pregnancy trial (n=110; HbA1c difference −0.17%, 95% CI −0.43 to 0.09; p=0.20) (PMC5713979). Whether this null finding reflects a true absence of benefit or simply insufficient statistical power remains uncertain. The evidence does not support the use of WHO growth standards for identifying LGA in T1DM pregnancies, given their failure to incorporate gestational age at birth and the high preterm delivery rates in this population (PMC7845036). The evidence does not support the conclusion that CGM significantly reduces maternal obstetric outcomes including pre-eclampsia, caesarean section, or preterm birth in T1DM pregnancies, as none of these outcomes reached statistical significance in the CONCEPTT trial, though directional trends favoured CGM for pre-eclampsia and early preterm birth (PMC5713979).

#### Future Research Directions

First, future trials should evaluate whether combining CGM with automated insulin delivery (closed-loop systems) produces greater reductions in LGA and other neonatal outcomes than CGM alone. The CONCEPTT trial authors noted that post-prandial hyperglycaemia — the "prandial problem" — persisted even in CGM users, and that automated insulin delivery might give women more confidence to administer larger pre-meal boluses with less fear of nocturnal hypoglycaemia (PMC5713979). Closed-loop insulin delivery systems are now being evaluated in T1DM pregnancy and represent the next frontier in glycaemic management for this population.

Second, future research should evaluate the impact of CGM on non-glycaemic determinants of LGA, including gestational weight gain, dietary composition, and lipid profiles. The CONCEPTT trial found no significant difference in gestational weight gain between CGM and control groups (13.1 vs. 13.7 kg, p=0.22), and both groups had weight gain above Institute of Medicine recommendations (PMC5713979). Interventions that combine CGM with structured dietary counselling, weight management support, and lipid monitoring may produce greater LGA reductions than CGM alone.

Third, the CONCEPTT planning pregnancy trial should be replicated with adequate statistical power to determine whether CGM use before conception — during the critical period of organogenesis — confers additional benefit beyond what is achieved by starting CGM in early pregnancy. A Belgian retrospective cohort study of 259 T1DM pregnancies over 22 years found that excessive gestational weight gain (OR 1.95, 95% CI 1.08–3.53, p=0.027), first-trimester HbA1c (OR 1.43, 95% CI 1.05–1.95, p=0.023), and third-trimester HbA1c (OR 1.70, 95% CI 1.07–2.71, p=0.026) were independent predictors of LGA in multivariable analysis, while pre-pregnancy BMI was not independently significant (PMC4946226). Given that only 15% of pregnant women with T1DM achieve target HbA1c levels during early pregnancy (PMC5713979), optimising glycaemic control before conception through CGM use may have important benefits that the underpowered CONCEPTT planning pregnancy trial (n=110) was unable to detect.

Fourth, future research should establish GDM-specific CGM cut-off values for TIR, TAR, and TBR, which are currently extrapolated from T1DM data (PMC12470761). The International Consensus has endorsed the same glycaemic targets for T1DM, T2DM, and GDM in pregnancy, but the physiological differences between these conditions — including different degrees of insulin resistance, different patterns of glycaemic variability, and different baseline glucose profiles — may warrant condition-specific targets. Large, prospective, multi-ethnic cohort studies integrating CGM data with clinical outcomes are needed to establish evidence-based, condition-specific CGM targets for pregnancy.

Fifth, the mechanistic role of circulating extracellular vesicles as biomarkers of vascular injury and predictors of neonatal outcomes in T1DM pregnancies warrants further investigation. In the CONCEPTT EV secondary analysis, higher baseline endothelial extracellular vesicle (EV) levels were independently associated with NICU admission, composite neonatal outcome, and hyperbilirubinaemia after adjustment for HbA1c and blood pressure (PMC8419913). If validated in larger cohorts, EV profiling in early pregnancy could identify women at highest risk of neonatal complications who might benefit most from intensive CGM-guided management.

Sixth, cost-effectiveness analyses of CGM implementation in T1DM pregnancy are urgently needed. The CONCEPTT trial demonstrated that CGM reduces NICU admissions (NNT=6) and infant hospital stay by approximately one day, which may substantially offset the cost of CGM devices and sensors. However, formal health-economic analyses comparing the costs of CGM implementation against the savings from reduced neonatal intensive care have not yet been published for the CONCEPTT population, and such analyses are essential for informing reimbursement decisions and guideline recommendations (PMC5713979).

Seventh, future research should evaluate CGM outcomes in more ethnically diverse T1DM pregnancy populations. The CONCEPTT trial was 87% European or Mediterranean in origin, and the CONCEPTT growth standards secondary analysis found that GROW and INTERGROWTH performed comparably in this predominantly European cohort but that the relative performance of these standards in other ethnic groups remains uncertain (PMC7845036). Given that T1DM prevalence and pregnancy outcomes vary across ethnic groups, and that growth standards may perform differently in non-European populations, ethnically diverse replication studies are needed.

Finally, the long-term outcomes of children born to mothers who used CGM during pregnancy — including rates of childhood obesity, impaired glucose tolerance, and neurodevelopmental outcomes — should be systematically evaluated. The CONCEPTT trial collected data only through the neonatal period, and the long-term consequences of the LGA reduction achieved by CGM for offspring metabolic health remain unknown. Given the evidence that intrauterine exposure to maternal hyperglycaemia programmes offspring metabolic risk through epigenetic mechanisms (PMC12429770, PMC12470761), and that LGA itself is associated with a 1.5-fold higher risk of childhood obesity and a 7.4-fold higher risk of type 2 diabetes (PMC11928751), the long-term offspring benefits of CGM-mediated LGA reduction may substantially exceed the short-term neonatal benefits captured in the CONCEPTT trial.

In summary, the CONCEPTT trial provides robust, high-quality evidence that real-time CGM significantly reduces LGA incidence in pregnant women with pre-existing type 1 diabetes compared with standard SMBG, with an absolute risk reduction of 16 percentage points and a number needed to treat of six. This finding, accompanied by significant reductions in NICU admission and neonatal hypoglycaemia, has appropriately led to guideline recommendations that CGM should be offered to all pregnant women with T1DM using intensive insulin therapy from the first trimester. However, the persistent high LGA rate even in CGM users (53%), the modest HbA1c improvement, and the contribution of non-glycaemic factors to fetal overgrowth collectively indicate that CGM is a necessary but not sufficient component of optimal T1DM pregnancy management, and that further advances in technology, multidisciplinary care, and our understanding of the full spectrum of determinants of fetal overgrowth will be required to meaningfully normalise pregnancy outcomes in this high-risk population.

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