Gestational Diabetes Mellitus

[2]

1. Definition: first separate GDM from overt diabetes

Modern classification distinguishes diabetes mellitus in pregnancy from GDM. WHO's live IRIS diagnostic authority classifies fasting plasma glucose ≥7.0 mmol/L, 2-hour plasma glucose after 75 g ≥11.1 mmol/L, or a random plasma glucose ≥11.1 mmol/L with classic symptoms as diabetes in pregnancy; absent unequivocal hyperglycaemia, confirm an abnormal diagnostic result. Lower pregnancy-specific hyperglycaemia may meet GDM criteria. HbA1c ≥6.5% can support recognition of pre-existing diabetes, but HbA1c alone is neither a universal booking screen nor the routine diagnostic test for GDM.[1]

The distinction matters because organogenesis has already occurred by routine GDM testing. Overt diabetes requires urgent specialist review for glucose treatment, retinal/renal assessment and fetal structural evaluation. Immediately review chronic medicines and, in line with NICE NG3, stop ACE inhibitors/ARBs and statins as soon as pregnancy is confirmed, substituting pregnancy-compatible treatment where needed.[15]

2. Who and when to test

Routine testing is usually performed at 24 to 28 weeks, when pregnancy-related insulin resistance is established. Strategy varies: WHO/IADPSG and many international services use a one-step 75 g OGTT; ACOG supports the US two-step strategy; NICE NG3 offers a 75 g OGTT to people with specified risk factors rather than using a 50 g challenge. The test selected by the local service determines both sampling and thresholds.[1][8][10]

Risk factors include previous GDM, previous macrosomic infant, higher BMI using locally appropriate ethnicity-sensitive thresholds, first-degree family history of diabetes and higher-risk ethnic background. NICE NG3 also uses glycosuria 2+ once or 1+ on two or more occasions as a reason to consider further testing. Risk factors never justify borrowing thresholds from another pathway.[9]

Early-pregnancy testing primarily seeks previously unrecognised pregestational diabetes. The thresholds, benefits and harms of diagnosing and treating milder GDM before 24 weeks remain less certain. TOBOGM found a modest improvement in a composite neonatal outcome with immediate treatment in a selected early-GDM population, but this does not make one early-GDM algorithm universal. If early testing is normal, people still eligible for routine testing are retested at 24 to 28 weeks according to local guidance.[10][11]

3. Three diagnostic pathways—keep each intact

Reader-visible authorities: WHO 2013 diagnostic classification, NICE NG3 recommendations, ADA Standards of Care 2026, and the ACOG 2024 screening update.[1][10][15]

The UK panel follows NICE NG3.[1][9]

[9]

The HAPO study included 25,505 pregnancies at 15 centres in nine countries and found continuous associations between maternal glucose and outcomes without an obvious natural threshold. Associations differed by endpoint: fasting glucose was strongest for high cord C-peptide, whereas 1-hour glucose was strongest for birthweight above the 90th centile. IADPSG converted those continuous data into pragmatic thresholds, so a diagnostic cut-off is a policy boundary rather than a biological cliff.[3][1]

4. Pathophysiology and outcomes

[2]

Placental hormones, adipokines and inflammatory mediators progressively reduce maternal insulin sensitivity. In normal pregnancy, beta-cell secretion rises to compensate. GDM develops when underlying beta-cell reserve and insulin sensitivity cannot meet that demand. This is a relative failure of compensation; placental hormones do not necessarily cause an absolute decline in insulin output.[2]

Maternal glucose crosses the placenta, whereas maternal insulin ordinarily does not. Fetal hyperglycaemia stimulates fetal insulin secretion. Fetal insulin is anabolic, increasing adiposity and organ growth; after cord clamping, persistent insulin with abrupt loss of maternal glucose creates neonatal hypoglycaemia risk.[2][3]

[3] [12]

Treatment changes outcomes, but cite the endpoint correctly. In ACHOIS, the serious perinatal composite was 1% with intervention versus 4% with routine care, with more induction but similar caesarean rates. In the MFMU mild-GDM trial, the primary composite was not significantly different (32.4% vs 37.0%), but treatment reduced large-for-gestational-age birth (7.1% vs 14.5%), birthweight above 4 kg (5.9% vs 14.3%), shoulder dystocia (1.5% vs 4.0%), caesarean birth (26.9% vs 33.8%) and combined pre-eclampsia/gestational hypertension (8.6% vs 13.6%).[4][5]

5. Initial management: nutrition, activity and monitoring

Management begins promptly with a diabetes-in-pregnancy team. Individualise medical nutrition therapy to cultural food patterns, adequate maternal/fetal nutrition and appropriate gestational weight gain; spread carbohydrate intake to reduce peaks and avoid severe restriction or ketosis. There is no universal carbohydrate percentage, calorie reduction or mandatory bedtime snack for every patient.[9][15]

If no obstetric contraindication exists, advise regular moderate physical activity—for example about 30 minutes on most days—with post-meal walking as a practical option. Teach self-monitoring at times that answer a treatment question, usually fasting and after meals, and review the pattern rather than reacting to one isolated value.[9][15]

HbA1c is not the routine diagnostic test for GDM and can miss postprandial hyperglycaemia. Continuous glucose monitoring may help selected patients, especially when insulin profiles are difficult to interpret, but it does not replace the named diagnostic test or establish one universal GDM time-in-range target.[15]

6. Pharmacotherapy is regional

Sources: ADA 2026 and NICE NG3.[6][9][15]

[9]

Metformin lowers hepatic glucose production and improves insulin sensitivity but crosses the placenta. In MiG, the composite neonatal outcome was similar (32.0% metformin vs 32.2% insulin); 46.3% allocated metformin required supplemental insulin, and treatment preference favoured metformin (76.6% vs 27.2%). MiG-TOFU provides actual GDM offspring follow-up, but heterogeneous 7-to-9-year findings do not establish complete long-term equivalence. Glyburide also crosses the placenta and meta-analysis found less favourable neonatal outcomes than insulin or metformin; it is not a default substitute.[6][7][12]

7. Fetal surveillance, growth and birth

Separate growth assessment from antenatal wellbeing testing. NICE NG3 offers ultrasound assessment of fetal growth and amniotic fluid at 28, 32 and 36 weeks. In US practice, medication-treated or poorly controlled GDM commonly prompts NST/BPP surveillance from about 32 weeks, while there is no consensus that well-controlled diet-only GDM requires routine testing before 40 weeks. Individualise for hypertension, growth abnormality, reduced movements and other obstetric indications.[9]

Delivery timing is also regional. NICE NG3 advises birth no later than 40+6 weeks for uncomplicated GDM and earlier birth when maternal or fetal complications arise. ACOG generally avoids delivery before 39+0 for well-controlled diet-only GDM and allows expectant management to 40+6; for well-controlled medication-treated GDM, 39+0 to 39+6 is usual. Poor control or complications require individualised earlier planning.[9]

Estimated fetal weight is imprecise. In a pregnancy complicated by diabetes, EFW ≥4,500 g is a threshold for counselling about the potential benefits and harms of scheduled caesarean birth—not an automatic operation. Mode of birth also depends on prior births, pelvis, labour progress, patient preferences and local guidance.[9]

8. Intrapartum care

Document a labour plan before birth. NICE NG3 recommends capillary glucose hourly during labour and birth, aiming for 4 to 7 mmol/L; use intravenous dextrose and insulin if glucose cannot be maintained in that range. This is not a mandate for a fixed 50-unit bag, dextrose rate or infusion in every insulin-treated patient. US protocols often target glucose below about 110 mg/dL, so follow the named local protocol and avoid maternal hypoglycaemia.[9]

Prepare for shoulder dystocia when fetal size is high, but do not attempt a prophylactic manoeuvre. Use standard team rehearsal and HELPERR if shoulder dystocia occurs. Active management of the third stage follows obstetric indication; GDM does not replace routine haemorrhage-risk assessment.[9]

9. Newborn care

Tell the neonatal team before birth. NICE NG3 advises skin-to-skin contact, feeding as soon as possible and within 30 minutes, then every 2 to 3 hours, with routine neonatal glucose testing at 2 to 4 hours. Separately, the BAPM 2024 neonatal framework specifies measuring glucose before the second feed (2 to 4 hours); subsequent timing and thresholds follow the local at-risk newborn pathway.[17]

Also assess temperature, respiratory status, jaundice and feeding. Do not delay escalation for jitteriness, lethargy, poor feeding, apnoea or seizures while debating one numerical threshold; follow the BAPM/local neonatal pathway.[17]

10. Postpartum, prevention and future pregnancy

Stop glucose-lowering medication used only for GDM after birth, check maternal glucose before discharge, and investigate persistent hyperglycaemia as possible diabetes, as specified in NICE NG3 and ADA 2026. Insulin needs fall rapidly after placental delivery, but never stop treatment automatically if overt diabetes is suspected or confirmed.[15]

[15]

A systematic review and meta-analysis found prior GDM associated with an almost 10-fold higher subsequent type 2 diabetes risk, while absolute risk varies with follow-up and population.[16] DPP/DPPOS data support structured lifestyle intervention and, for selected people with prediabetes and prior GDM, metformin after shared decision-making. Avoid promising that one fixed percentage of weight loss “halves” every person's risk.[13][14]

Support breastfeeding, weight optimisation without crash dieting, regular activity and cardiovascular-risk review. Discuss contraception and pregnancy timing. Before another pregnancy, review glucose status and medicines; in the next pregnancy arrange early testing for overt diabetes and repeat routine 24-to-28-week testing if the early result is normal, following NICE NG3 or the named local pathway. Prior GDM or PCOS does not justify universal metformin prophylaxis during pregnancy.[15]

11. Exam pearls

• First say which regional pathway you are using; never combine WHO/IADPSG, ACOG and NICE NG3.[1][10]
• WHO/IADPSG: 75 g, any one of ≥5.1/≥10.0/≥8.5 mmol/L.[1]
• ACOG: 50 g screen, then 100 g 3-hour OGTT; Carpenter-Coustan usually needs two abnormal values.[9]
• NICE NG3: risk factors directly to 75 g OGTT; fasting ≥5.6 or 2-hour ≥7.8 mmol/L.
• NICE 2-hour treatment target is <6.4 mmol/L; ADA/ACOG 2-hour target is <6.7 mmol/L. Sources: NICE NG3 and ADA 2026.[9][15]
• ADA/ACOG prefer insulin; NICE NG3 usually offers metformin after a short lifestyle trial, with immediate-insulin exceptions.[9]
• EFW ≥4,500 g in diabetes triggers counselling about scheduled caesarean, not an automatic operation.[9]
• BAPM 2024: neonatal 40% dextrose gel 200 mg/kg is approximately 0.5 mL/kg; IV glucose follows the neonatal protocol.[17]
• ADA 2026: 75 g OGTT at 4–12 weeks, then every 1–3 years. NICE NG3: fasting glucose at 6–13 weeks, then annual HbA1c if normal.[15]

[9] [15]

The mnemonic's UK and US branches are sourced to NICE NG3 and ADA 2026.[15]

References

1. [1]Metzger BE, Gabbe SG, Persson B, et al. International association of diabetes and pregnancy study groups recommendations on the diagnosis and classification of hyperglycemia in pregnancy Diabetes Care, 2010.PMID 20190296
2. [2]Plows JF, Stanley JL, Baker PN, Reynolds CM, Vickers MH. The Pathophysiology of Gestational Diabetes Mellitus Int J Mol Sci, 2018.PMID 30373146
3. [3]Metzger BE, Lowe LP, Dyer AR, et al. Hyperglycemia and adverse pregnancy outcomes N Engl J Med, 2008.PMID 18463375
4. [4]Crowther CA, Hiller JE, Moss JR, McPhee AJ, Jeffries WS, Robinson JS. Effect of treatment of gestational diabetes mellitus on pregnancy outcomes N Engl J Med, 2005.PMID 15951574
5. [5]Landon MB, Spong CY, Thom E, et al. A multicenter, randomized trial of treatment for mild gestational diabetes N Engl J Med, 2009.PMID 19797280
6. [6]Rowan JA, Hague WM, Gao W, Battin MR, Moore MP. Metformin versus insulin for the treatment of gestational diabetes N Engl J Med, 2008.PMID 18463376
7. [7]Balsells M, García-Patterson A, Solà I, et al. Glibenclamide, metformin, and insulin for the treatment of gestational diabetes: a systematic review and meta-analysis BMJ, 2015.PMID 25609400
8. [8]Sacks DA, Hadden DR, Maresh M, et al. Frequency of gestational diabetes mellitus at collaborating centers based on IADPSG consensus panel-recommended criteria Diabetes Care, 2012.PMID 22355019
9. [9]ACOG. ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus Obstet Gynecol, 2018.PMID 29370047
10. [10]ACOG. ACOG Clinical Practice Update: Screening for Gestational and Pregestational Diabetes in Pregnancy and Postpartum Obstet Gynecol, 2024.PMID 42131962
11. [11]Simmons D, Immanuel J, Hague WM, et al. Treatment of Gestational Diabetes Mellitus Diagnosed Early in Pregnancy N Engl J Med, 2023.PMID 37144983
12. [12]Rowan JA, Rush EC, Plank LD, et al. Metformin in gestational diabetes: the offspring follow-up (MiG TOFU): body composition and metabolic outcomes at 7-9 years of age BMJ Open Diabetes Res Care, 2018.PMID 29682291
13. [13]Ratner RE, Christophi CA, Metzger BE, et al. Prevention of diabetes in women with a history of gestational diabetes: effects of metformin and lifestyle interventions J Clin Endocrinol Metab, 2008.PMID 18826999
14. [14]Aroda VR, Christophi CA, Edelstein SL, et al. The effect of lifestyle intervention and metformin on preventing or delaying diabetes among women with and without gestational diabetes: the Diabetes Prevention Program outcomes study 10-year follow-up J Clin Endocrinol Metab, 2015.PMID 25706240
15. [15]American Diabetes Association Professional Practice Committee for Diabetes. 15. Management of Diabetes in Pregnancy: Standards of Care in Diabetes—2026 Diabetes Care, 2026.PMID 41358885
16. [16]Vounzoulaki E, Khunti K, Abner SC, Tan BK, Davies MJ, Gillies CL. Progression to type 2 diabetes in women with a known history of gestational diabetes: systematic review and meta-analysis BMJ, 2020.PMID 32404325
17. [17]Harris DL, Weston PJ, Signal M, Chase JG, Harding JE. Dextrose gel for neonatal hypoglycaemia (the Sugar Babies Study): a randomised, double-blind, placebo-controlled trial Lancet, 2013.PMID 24075361