The full reasoning pathway — hyperglycaemia in pregnancy is either pre-existing (overt) diabetes or true gestational diabetes. Diagnose GDM on a 75 g OGTT, treat to tight glucose targets to protect the baby, and never forget the postnatal review — half go on to type 2 diabetes.StartDecisionInvestigateActionReferStop / Admit
PresentationPregnant — risk factors or glycosuria
BMI >30, previous baby ≥4.5 kg, previous GDM, first-degree relative with diabetes, high-risk ethnicity, or 2+/persistent glycosuria.
Step 1 · SafetyOvert diabetes at booking?
Booking HbA1c ≥48 mmol/mol or random/fasting glucose in the diabetic range = pre-existing (often undiagnosed type 2) — manage as diabetes in pregnancy, not GDM.
YES
ReferJoint diabetes-antenatal clinic
Treat as pre-existing diabetes: optimise glucose, high-dose folic acid 5 mg, retinal & renal screen, aspirin for pre-eclampsia.
NO
Investigate75 g OGTT at 24–28 weeks
Previous GDM → test early (self-monitoring or OGTT soon after booking) and repeat at 24–28 weeks if normal.
GDM if fasting ≥5.6 or 2-hr ≥7.8
DecisionFasting glucose at diagnosis?
Drives the starting treatment — diet trial versus straight to insulin.
Fasting <7
ActionDiet & exercise trial 1–2 weeks
If targets not met → add metformin → add insulin. Consider insulin sooner if macrosomia/polyhydramnios.
Fasting ≥7
ActionInsulin now (± metformin)
Too high for a diet trial — start insulin immediately alongside lifestyle.
Action · SurveillanceTargets + growth scans
Fasting <5.3, 1-hr <7.8, 2-hr <6.4. Growth/liquor scans 28/32/36 wks. Plan birth by 40+6.
Postnatal — do not missStop drugs + retest
Stop hypoglycaemics after delivery. Fasting glucose 6–13 weeks postpartum (or HbA1c after 13 weeks), then annual HbA1c — ~50% develop type 2 diabetes.
Step 8 · lifestyle & modifiable factors
Step 8 · Lifestyle & self-managementFirst-line and lifelong
Dietary change (low-GI, portion/carbohydrate awareness with a dietitian) and regular physical activity are first-line for GDM. Capillary glucose self-monitoring to targets; weight management between pregnancies; stop smoking. Encourage breastfeeding (lowers maternal & infant future diabetes risk). Reinforce that lifestyle reduces the ~50% progression to type 2 diabetes.
Step 9 · monitoring & safety-net
Step 9 · Monitoring & safety-netSurveillance & the never-miss postnatal test
Growth/liquor scans (28/32/36 wks) and glucose to target; escalate to insulin promptly if not met or macrosomia/polyhydramnios. Postnatal fasting glucose at 6–13 weeks then annual HbA1c — the most-missed long-term action. Counsel re: early OGTT in future pregnancies. Seek urgent review for reduced fetal movements, signs of pre-eclampsia, or symptomatic hyper/hypoglycaemia.
⚠️ Two things people miss: a high booking HbA1c (≥48) is pre-existing diabetes, not GDM — it needs pre-pregnancy-style care and pre-eclampsia aspirin. And the postnatal glucose test is forgotten in a huge proportion of women — it is the single most important long-term action, because gestational diabetes is a powerful predictor of future type 2 diabetes.
1
Safety
Red Flags — Overt Diabetes, DKA, Pre-eclampsia & Fetal Compromise
Overt (pre-existing) diabetes Booking HbA1c ≥48 mmol/mol, or fasting/random glucose in the diabetic range, means undiagnosed type 1 or type 2 — not GDM. Manage as diabetes in pregnancy: tighter targets, 5 mg folic acid, retinal/renal screening, and low-dose aspirin from 12 weeks for pre-eclampsia.
Diabetic ketoacidosis Can occur in type 1 (or rarely type 2) at lower glucose levels in pregnancy ("euglycaemic DKA"). Vomiting, abdominal pain, thirst, breathlessness, ketones → same-day emergency obstetric/medical admission. A genuine threat to mother and fetus.
Pre-eclampsia GDM and pre-existing diabetes both raise the risk. Watch BP, proteinuria, headache, visual disturbance, epigastric pain, brisk reflexes — urgent assessment. Offer aspirin prophylaxis in those at risk.
Reduced fetal movements / polyhydramnios / macrosomia Markers of fetal compromise and of poor glycaemic control; diabetes carries a higher stillbirth risk. Reduced movements → same-day assessment. Polyhydramnios or accelerating growth → review control and scan.
Severe maternal hypoglycaemia In women on insulin or a sulfonylurea — risk rises in the first trimester and with tight targets. Counsel on recognition, treatment, and DVLA/driving rules where relevant.
Shoulder dystocia risk at delivery Macrosomia from poor control predisposes to shoulder dystocia and neonatal injury — informs the timing and mode-of-birth discussion with obstetrics.
The first safety job is to separate true gestational diabetes (glucose intolerance that first appears in pregnancy) from pre-existing diabetes that has only now been detected, because the latter needs more intensive care and pre-eclampsia prophylaxis. The acute emergencies — DKA, which can be euglycaemic in pregnancy, and pre-eclampsia — threaten both mother and baby. And throughout, the unspoken red flag is the raised stillbirth risk of poorly controlled diabetes, which is precisely why glucose targets in pregnancy are so tight and fetal surveillance so structured.
2
Diagnose
Who to Screen — Risk Factors & Timing
Risk factors (any one → OGTT)
BMI >30 kg/m²; previous macrosomic baby ≥4.5 kg; previous gestational diabetes; first-degree relative with diabetes; high-risk ethnicity (South Asian, Black Caribbean, Middle Eastern).
Previous GDM
Offer early self-monitoring of blood glucose or a 75 g OGTT as soon as possible after booking, and a further OGTT at 24–28 weeks if the first is normal.
All other risk factors
75 g OGTT at 24–28 weeks.
Glycosuria in pregnancy
2+ on one occasion or 1+ on two occasions on routine testing → consider further testing for GDM.
Symptoms
Most GDM is asymptomatic and found on screening. Thirst, polyuria, fatigue or recurrent thrush should prompt glucose testing at any gestation.
The UK uses risk-factor-based screening rather than universal OGTT. Identifying any single risk factor triggers a 75 g oral glucose tolerance test, timed at 24–28 weeks for most women but brought forward for those with previous gestational diabetes, who have the highest recurrence rate. Because the condition is usually silent, the screening question — "does this woman have a risk factor?" — is what makes the diagnosis, not symptoms.
Booking HbA1c ≥48 mmol/mol, or fasting ≥7.0 / random ≥11.1 with symptoms — manage as pre-existing diabetes.
HbA1c in pregnancy
Not used to diagnose GDM (red-cell turnover changes make it unreliable) — useful at booking to detect overt diabetes and to assess later risk.
Communicate the diagnosis well
Explain that good control greatly reduces risks, that it often resolves after birth, and that it is not caused by anything she did wrong — addressing guilt and engaging her early improves outcomes.
The two diagnostic numbers to commit to memory are a fasting glucose of 5.6 mmol/L or above and a two-hour glucose of 7.8 mmol/L or above on the 75 g OGTT — either one alone makes the diagnosis. HbA1c does not diagnose gestational diabetes because pregnancy alters red-cell turnover, but a booking HbA1c of 48 or more identifies overt diabetes that predates the pregnancy. How the diagnosis is delivered matters clinically: a woman who understands the rationale and feels supported is far more likely to achieve the tight targets that protect her baby.
4
Diagnose
Differential — GDM vs Pre-existing vs Transient
Gestational diabetes
Glucose intolerance first recognised in pregnancy, typically 2nd/3rd trimester, resolving after delivery. The default when OGTT thresholds are met and booking HbA1c was normal.
Pre-existing type 2 diabetes
Often undiagnosed before pregnancy; suggested by high booking HbA1c, obesity, strong risk factors, or hyperglycaemia in the first trimester.
Type 1 diabetes (new)
Younger, lean, rapid osmotic symptoms, ketosis-prone — needs urgent specialist care; do not mislabel as GDM.
Transient stress / steroid hyperglycaemia
Antenatal corticosteroids (for fetal lung maturity) transiently raise glucose and may need short-term insulin cover — not a new diagnosis of GDM.
Renal glycosuria
Glucose in the urine with normal blood glucose — a lowered renal threshold, common in pregnancy and benign; confirm with blood glucose/OGTT before labelling.
Most hyperglycaemia detected in pregnancy is gestational diabetes, but the differential matters because pre-existing type 2 diabetes needs more intensive management and a different risk conversation, and new type 1 diabetes is a specialist emergency. Two common traps are steroid-induced hyperglycaemia after antenatal corticosteroids, which is transient, and pregnancy-related renal glycosuria, which reflects a lowered renal threshold rather than high blood glucose — both are confirmed by measuring blood glucose rather than relying on the dipstick.
5
Diagnose
Monitoring & Fetal Surveillance
Self-monitoring of blood glucose
Fasting and 1-hour post-meal capillary glucose. Targets: fasting <5.3, 1-hour post-meal <7.8 (or 2-hour <6.4) mmol/L; keep above 4 to avoid hypoglycaemia if on insulin/sulfonylurea.
Growth & liquor scans
Ultrasound for fetal growth and amniotic fluid at 28, 32 and 36 weeks; more often if concerns about macrosomia or polyhydramnios.
Pre-eclampsia surveillance
Regular BP and urinalysis; aspirin prophylaxis where indicated.
Glucose at antenatal contacts
Review self-monitoring records at each visit; escalate treatment promptly if targets are not met.
Management is driven by capillary glucose self-monitoring against deliberately tight targets — a fasting value below 5.3 and a one-hour post-meal value below 7.8 — because maternal hyperglycaemia drives fetal overgrowth and its complications. Serial growth and liquor-volume scans at 28, 32 and 36 weeks detect macrosomia and polyhydramnios, which both signal suboptimal control and inform the timing and mode of birth.
6
Refer
Referral & Shared Care
Joint diabetes-antenatal clinic
Refer all women with GDM (ideally seen within 1 week of diagnosis) for combined obstetric and diabetes-team care — dietitian, diabetes specialist nurse/midwife.
Manage as pre-existing diabetes
Booking HbA1c ≥48 or first-trimester hyperglycaemia → pre-existing diabetes pathway with tighter targets, 5 mg folic acid, retinal/renal screening.
Same-day
Suspected DKA, severe pre-eclampsia, or reduced fetal movements → urgent obstetric assessment.
Neonatal planning
Deliver in a unit with neonatal facilities; alert the neonatal team — risk of neonatal hypoglycaemia, jaundice and respiratory distress.
Anaesthetic / obstetric
Macrosomia or other complications → consultant-led birth planning (timing, mode, intrapartum glucose control).
Gestational diabetes is managed in a joint diabetes-antenatal clinic, and prompt referral — within about a week of diagnosis — gets dietary advice, monitoring and, where needed, medication started during the window when they most influence fetal growth. The general practitioner's role is to recognise the diagnosis, distinguish pre-existing diabetes, initiate safe early advice, and act urgently on the obstetric and metabolic emergencies while specialist care is arranged.
7
Treat
Stepwise Glucose Control — Diet → Metformin → Insulin
Fasting <7 mmol/L, no complications
Diet & exercise trial
1–2 week trial of dietary change and physical activity. If glucose targets are not met → step up.
Targets not met after trial
Add metformin
First-line oral agent in pregnancy. If metformin is not tolerated/declined and insulin declined → glibenclamide is an alternative.
Fasting ≥7, or ≥6 with macrosomia/hydramnios, or targets still unmet
Insulin (± metformin)
Start insulin immediately if fasting ≥7 at diagnosis. Add to metformin when diet + metformin are insufficient.
Step 1Diet & exercise — low-glycaemic-index carbohydrates, portion control, 30 minutes of walking after meals. Refer to a dietitian. Review glucose in 1–2 weeks.
Step 2Metformin — first-line oral medication; safe and effective in pregnancy. Titrate to tolerance.
Step 3Insulin — added when targets remain unmet, or started first-line when fasting glucose is ≥7 mmol/L at diagnosis or there is fetal macrosomia/polyhydramnios. Counsel on hypoglycaemia.
AlternativeGlibenclamide — for women who cannot tolerate metformin and decline insulin.
Treatment escalates in steps but the starting rung depends on the fasting glucose. A woman whose fasting glucose is below 7 with no fetal complications earns a one-to-two-week trial of diet and exercise; if that fails to reach target, metformin is the first-line oral drug and insulin is added next. A fasting glucose of 7 or more at diagnosis is too high for a lifestyle trial and warrants insulin straight away. Glibenclamide is reserved for the woman who cannot take metformin and declines insulin.
8
Lifestyle
Diet, Activity & Self-Management
Carbohydrate quality Choose low-glycaemic-index carbohydrates, control portions, and spread intake across meals and snacks to blunt post-meal peaks. Dietitian input is part of standard care.
Physical activity Regular moderate activity — a 20–30 minute walk after meals is especially effective at lowering post-prandial glucose. Encourage unless an obstetric contraindication exists.
Weight & gestational gain Avoid excessive gestational weight gain; if BMI >27, advice on healthy eating reduces risk. Weight loss is not pursued during pregnancy.
Self-monitoring skills Teach reliable capillary testing, recognising and treating hypoglycaemia (if on insulin/sulfonylurea), and keeping a glucose diary to guide reviews.
Breastfeeding Encourage — it benefits maternal glucose metabolism and reduces the child's later obesity/diabetes risk. Metformin and insulin are compatible with breastfeeding.
Future-pregnancy planning Explain the high recurrence rate, the value of pre-conception weight optimisation, and early testing in any future pregnancy.
Lifestyle is genuinely therapeutic in gestational diabetes — many women reach target on diet and activity alone. The highest-yield measures are low-glycaemic-index carbohydrate choices, portion control, and a short walk after meals to flatten post-prandial peaks. Self-monitoring competence and hypoglycaemia awareness keep treatment safe, and breastfeeding plus pre-conception planning extend the benefit well beyond the index pregnancy for both mother and child.
9
Safety
Birth, Postnatal Care & Long-Term Follow-Up
Timing of birth
Offer elective birth no later than 40+6 weeks in GDM; earlier if macrosomia, poor control, or other complications — agreed with obstetrics.
Immediately after delivery
Stop all glucose-lowering medication right after birth. Feed the baby early and monitor neonatal blood glucose — risk of neonatal hypoglycaemia, jaundice and respiratory distress.
Postnatal maternal testing
Fasting plasma glucose at 6–13 weeks postpartum; if missed, HbA1c after 13 weeks. Then an annual HbA1c — around half develop type 2 diabetes, many within 5 years.
Lifestyle & prevention
Weight management, diet and activity advice reduce progression to type 2 diabetes. Address pre-conception care for future pregnancies.
Safety-net
Symptoms of hyperglycaemia (thirst, polyuria, weight loss) postnatally → test for diabetes. Reduced fetal movements antenatally → same-day assessment.
Recurrence
High recurrence in future pregnancies — arrange early self-monitoring or OGTT soon after the next booking.
Two safety priorities bracket the birth. Around delivery, glucose-lowering drugs are stopped immediately and the neonate is monitored for hypoglycaemia, the commonest neonatal complication. Then comes the most frequently neglected step in the whole pathway: the postnatal maternal glucose check at 6–13 weeks (or HbA1c thereafter) followed by annual testing, because gestational diabetes confers a very high lifetime risk of type 2 diabetes and is a powerful, actionable prevention opportunity. Recurrence in future pregnancies is high, so early retesting is planned in advance.
Educational use only. Based on NICE NG3 (Diabetes in pregnancy, OGTT thresholds fasting ≥5.6 / 2-hour ≥7.8 mmol/L), NICE CKS Diabetes in pregnancy, and joint diabetes-antenatal care standards. Always adapt to individual patient context.