Gestational Diabetes Mellitus — Branching Viva

Start node — first classify the hyperglycaemia

Reader-visible authorities for every branch: WHO IRIS diagnostic classification, NICE NG3, ADA 2026, ACOG 2024, and BAPM 2024 neonatal guidance.^[1][4][7][8]

Examiner: “A pregnant patient has an abnormal glucose result. What is your first question?”^[1][4]

Candidate: “I first ask whether the result meets overt-diabetes thresholds and which named regional diagnostic pathway was used. GDM is pregnancy-specific hyperglycaemia below overt-diabetes thresholds; I will not mix WHO/IADPSG, ACOG and NICE NG3 criteria.”^[1][4]

Branch A — if the value is in the diabetes range

Examiner: “Fasting plasma glucose is 7.2 mmol/L at the first prenatal visit. What now?”^[1][4]

Candidate: “Fasting ≥7.0 mmol/L meets the WHO diabetes-in-pregnancy threshold. If there is no unequivocal hyperglycaemia, I confirm the abnormal diagnostic result, assess symptoms/ketones when indicated, and refer urgently through the pre-existing-diabetes pathway. I immediately review medicines and stop ACE inhibitors/ARBs and statins in line with NICE NG3, substituting pregnancy-compatible treatment. HbA1c can help identify chronic disease but is not a universal GDM screen.”^[1][7]

Examiner probe: “Why not call this GDM?”^[1]

Candidate: “Because overt diabetes carries different maternal organ and fetal structural implications. Calling every first-detected abnormality GDM hides probable pre-existing disease.”^[1]

Branch B — if early testing is abnormal but below overt-diabetes thresholds

Examiner: “The patient is 11 weeks and values are below overt-diabetes thresholds. Is the evidence settled?”^[4]

Candidate: “No. Early testing primarily looks for undiagnosed pregestational diabetes. Benefits and thresholds for milder early GDM remain less certain, so I follow the local protocol, discuss uncertainty and repeat routine 24-to-28-week testing if the early test is normal and the patient remains eligible.”^[4]

Diagnostic branch — examiner chooses a region

Branch C1 — WHO/IADPSG

Examiner: “Give the one-step criteria.”^[1]

Candidate: “On a fasting 75 g OGTT, any one value diagnoses GDM: fasting ≥5.1, 1 hour ≥10.0, or 2 hours ≥8.5 mmol/L, provided overt-diabetes thresholds are not met.”^[1]

Branch C2 — US ACOG two-step

Examiner: “Now use the US two-step pathway.”^[3][4]

Candidate: “First perform a non-fasting 50 g 1-hour glucose challenge. A positive local screen leads to a fasting 100 g 3-hour OGTT. Carpenter-Coustan thresholds are fasting 5.3, 1 hour 10.0, 2 hours 8.6, and 3 hours 7.8 mmol/L, usually requiring two or more abnormal values.”^[3][4]

Branch C3 — NICE

Examiner: “What does NICE do?”^[1]

Candidate: “NICE NG3 uses risk factors to select patients directly for a fasting 75 g 2-hour OGTT; it does not insert a 50 g challenge. GDM is fasting ≥5.6 mmol/L or 2 hours ≥7.8 mmol/L.”^[1]

Examiner probe: “What glycosuria pattern prompts further testing under NICE?”^[1]

Candidate: “NICE NG3 specifies glycosuria 2+ once or 1+ on two or more occasions.”^[1]

Mechanism branch

Examiner: “Explain GDM from placenta to newborn.”^[2]

Candidate: “Pregnancy signals increase maternal insulin resistance. A susceptible beta cell cannot increase insulin secretion enough; this is inadequate compensation, not an obligatory absolute fall. Maternal glucose crosses the placenta while maternal insulin ordinarily does not. Fetal insulin then promotes adiposity and organ growth, and persistent fetal insulin after cord clamping creates neonatal hypoglycaemia risk.”^[2]

Management branch — response to lifestyle determines the next node

Branch D1 — targets met without medication

Examiner: “Values are at target with nutrition and activity. What next?”^[3][7]

Candidate: “Continue individualised adequate nutrition, moderate activity if obstetrically safe, and fasting/post-meal monitoring. I avoid severe carbohydrate restriction, ketosis and a fixed weight-loss prescription during pregnancy.”^[3][7]

Examiner probe: “Give targets, but name the guideline.”^[3][7]

Candidate: “ADA/ACOG: fasting <5.3, 1 hour <7.8, 2 hours <6.7 mmol/L. NICE NG3: fasting <5.3, 1 hour <7.8, but 2 hours <6.4 mmol/L, if achievable without problematic hypoglycaemia.”^[3][7]

Branch D2 — repeated values remain high in US practice

Examiner: “Lifestyle is insufficient. The unit follows ADA/ACOG.”^[3][7]

Candidate: “Insulin is preferred. I match basal insulin to fasting elevation and mealtime insulin to post-meal elevation, individualising dose and titration. Metformin and glyburide cross the placenta and are not preferred first-line agents. ADA 2026 advises avoiding metformin with hypertension/pre-eclampsia or fetal-growth-restriction/placental-insufficiency risk.”^[3][7]

Branch D3 — repeated values remain high in NICE practice

Examiner: “The unit follows NICE.”^[5][7]

Candidate: “NICE NG3 offers metformin after 1 to 2 weeks without target control; use insulin if metformin is contraindicated or unacceptable, and add insulin if metformin remains insufficient. Offer immediate insulin, with or without metformin, at fasting ≥7.0 mmol/L; consider it at 6.0–6.9 with macrosomia or hydramnios. Before extrapolating this UK sequence, note the ADA 2026 caution to avoid metformin with hypertension/pre-eclampsia or fetal-growth-restriction/placental-insufficiency risk.”^[5][7]

Surveillance and birth branch

Branch E1 — well-controlled diet-only GDM

Examiner: “Does every patient need NSTs from 32 weeks and induction at 38 weeks?”^[3][4]

Candidate: “No. In US practice there is no consensus that well-controlled diet-only GDM needs routine antenatal testing before 40 weeks. ACOG generally avoids birth before 39 weeks and may await 40+6. NICE NG3 offers growth/amniotic-fluid scans at 28, 32 and 36 weeks and advises birth no later than 40+6 for uncomplicated GDM.”^[3]

Branch E2 — medication-treated or poorly controlled

Examiner: “What changes?”^[3][4]

Candidate: “Medication-treated or poorly controlled disease commonly prompts US antenatal testing from around 32 weeks. Well-controlled medication-treated GDM is usually delivered at 39+0 to 39+6 under ACOG; poor control, hypertension, abnormal growth or fluid requires individualised earlier planning.”^[3][4]

Examiner probe: “Estimated fetal weight is 4,550 g.”^[3]

Candidate: “In diabetes, EFW ≥4,500 g prompts counselling about scheduled caesarean benefits and harms. Ultrasound weight is imprecise, so this is not an automatic operation.”^[3]

Labour branch

Examiner: “The NICE patient is in labour. Give the glucose plan.”^[3]

Candidate: “Under NICE NG3, check capillary glucose hourly and aim for 4 to 7 mmol/L. Use intravenous dextrose and insulin if glucose cannot remain in range. I do not prescribe a universal infusion concentration or rate for every patient; I use the maternity protocol and avoid hypoglycaemia.”^[3]

Newborn branch — clinical state determines treatment

Branch F1 — prevention and screening

Examiner: “The baby looks well. What do you do first?”^[8]

Candidate: “NICE NG3 supports skin-to-skin care, feeding as soon as possible and within 30 minutes, then every 2 to 3 hours, with routine glucose testing at 2 to 4 hours. The BAPM 2024 framework separately specifies testing before the second feed (2 to 4 hours); then follow the local at-risk pathway.”^[8]

Branch F2 — asymptomatic low glucose under a gel protocol

Examiner: “Using the BAPM 2024 neonatal framework, calculate 40% dextrose gel 200 mg/kg.”^[8]

Candidate: “Forty-percent gel contains about 400 mg/mL, so 200 mg/kg is approximately 0.5 mL/kg buccally. I support a feed and recheck at the protocol-defined interval.”^[8]

Branch F3 — symptomatic or persistent low glucose

Examiner: “The baby is jittery and cannot feed.”^[8]

Candidate: “Escalate immediately to the neonatal team for protocolised IV glucose and investigation under the BAPM/local neonatal pathway; do not delay for another feed and do not quote one universal IV bolus volume without the neonatal guideline.”^[8]

Postpartum branch — examiner chooses a region

Examiner: “What happens to maternal medication immediately after birth?”^[7]

Candidate: “Under ADA 2026 and NICE NG3, stop medication used only for GDM, check maternal glucose before discharge, and investigate persistent hyperglycaemia as diabetes. Do not automatically stop treatment if overt diabetes is suspected.”^[7]

Branch G1 — ADA

Examiner: “US follow-up?”^[7]

Candidate: “ADA 2026 recommends a fasting 75 g OGTT at 4 to 12 weeks, preferred to HbA1c early postpartum; if normal, continue lifelong screening every 1 to 3 years.”^[7] “Offer intensive lifestyle support, and discuss metformin only for selected people with prediabetes/prior GDM.”^[6]

Branch G2 — NICE

Examiner: “UK follow-up?”^[7]

Candidate: “NICE NG3 recommends fasting plasma glucose at 6 to 13 weeks. After 13 weeks offer fasting glucose; use HbA1c only if fasting testing is not possible. NICE does not routinely offer a postpartum OGTT. If normal, offer annual HbA1c.”^[7]

Closing probe — future pregnancy

Examiner: “What preventive advice completes the viva?”^[6][7]

Candidate: “Support breastfeeding, sustainable weight optimisation, activity and cardiovascular-risk review; discuss contraception and pregnancy timing. Before another pregnancy review glucose and medicines, test early for overt diabetes, and repeat 24-to-28-week testing if early results are normal under NICE NG3 or the named local pathway. Prior GDM or PCOS is not a universal indication for metformin prophylaxis in pregnancy.”^[6][7]

References8ShowHide

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]ACOG. ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus Obstet Gynecol, 2018.PMID 29370047
4. [4]ACOG. ACOG Clinical Practice Update: Screening for Gestational and Pregestational Diabetes in Pregnancy and Postpartum Obstet Gynecol, 2024.PMID 42131962
5. [5]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
6. [6]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
7. [7]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
8. [8]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