Pregnancy · GDM
OGTT — Glucose Tolerance Test in Pregnancy
The fasting + glucose drink + blood test that screens for gestational diabetes at 24-28 weeks (or earlier if you've had GDM before). UK NICE NG3 thresholds, what positive means for the rest of pregnancy, what to eat, and birth plan.
Last reviewed 2 June 2026
Gestational diabetes — OGTT interpreter
75 g (IADPSG) or 100 g (Carpenter-Coustan) OGTT
Diagnostic standard
Units
mg/dL
mg/dL
mg/dL
Enter at least one plasma glucose value to interpret.
Educational tool only — not medical advice. IADPSG (one-step) is the preferred standard in WHO 2013, ADA, NICE; ACOG accepts both. GDM diagnosis triggers: dietary counselling, glucose monitoring, exercise prescription, and (for ~30 %) metformin or insulin. Treatment significantly reduces macrosomia, shoulder dystocia, and neonatal hypoglycaemia risk (HAPO trial / ACHOIS / MFMU 2009).
What does this mean?
GDM is screened for around 24–28 weeks and earlier (booking) in higher-risk women. The HAPO study (NEJM 2008) showed adverse pregnancy outcomes rise continuously with maternal glucose, so any single elevated value on a 75 g OGTT (fasting ≥ 92, 1 h ≥ 180, 2 h ≥ 153 mg/dL) meets the IADPSG definition. About 14 % of pregnancies worldwide (IDF Atlas) — more in Asian and Hispanic populations. First- line treatment is medical nutrition therapy + 30 min walking after meals; ~70 % achieve targets this way. If not, add metformin (safe in pregnancy per MiG trial 2008 and NICE NG3) or move to insulin. Diagnosed GDM raises lifetime type-2 diabetes risk; a 6–12 week postnatal OGTT is recommended (ADA, NICE), then lifestyle surveillance.
What is the OGTT?
Oral Glucose Tolerance Test — screens for gestational diabetes (GDM). Affects ~1 in 6 pregnancies UK.
Procedure:
- Fast overnight (10-12 hours, water OK).
- Blood drawn fasting.
- Drink 75g glucose drink (very sweet).
- Blood drawn at 1 hour + 2 hours.
Any one result above threshold = GDM diagnosed.
UK NICE NG3 thresholds (75g one-step)
- Fasting ≥5.6 mmol/L (101 mg/dL).
- 1 hour ≥10.0 mmol/L (180 mg/dL).
- 2 hours ≥7.8 mmol/L (140 mg/dL).
ANY ONE value above threshold = GDM.
Who needs it (NHS)?
Risk factors (any one triggers OGTT):
- BMI ≥30.
- Previous baby ≥4.5 kg.
- Previous GDM in any pregnancy.
- Family history of diabetes (parent / sibling).
- Ethnicity at higher risk (South Asian, Middle Eastern, Black, Hispanic).
- Previous unexplained stillbirth.
Previous GDM = offered earlier (16-18 weeks); repeat at 24-28 if negative.
If positive (GDM diagnosed)
- Dietitian referral — low-GI eating.
- Home glucose monitoring 4x/day.
- Metformin if diet alone not enough (now NHS first-line second step).
- Insulin if metformin not enough.
- Growth scans at 28, 32, 36 weeks.
- Delivery plan 39-40 weeks (or earlier with concerns).
- Neonatal hypoglycaemia checks for baby.
- Postpartum 6-12 week OGTT; annual HbA1c.
What to eat with GDM
Low-GI principles:
- Eat: protein every meal (eggs, fish, meat, beans); green vegetables; whole grains; berries; nuts; Greek yoghurt; cheese; healthy fats.
- Limit: white bread, white rice, sugary cereals, juice, sweets, soft drinks, fruit smoothies, pastries.
- Smaller portions, every 3 hours.
- Protein + fibre + healthy fat with every carb.
Glucose drink — if you can’t face it
The drink is very sweet (75g sugar dissolved in water). Strategies:
- Very cold helps.
- Sip slowly over 5 minutes (within 5-15 min window).
- Ginger / mint after.
- Bucket nearby — vomit invalidates result.
Alternatives if unable: HbA1c (less accurate), random glucose, CGM (continuous glucose monitor) for 1-2 weeks.
Will baby be affected?
Most babies of well-controlled GDM mothers: healthy.
Poorly controlled GDM risks:
- Big baby (LGA / macrosomia) — birth trauma, shoulder dystocia.
- Neonatal hypoglycaemia.
- Jaundice.
- Respiratory distress.
- NICU stay (briefly).
Good control minimises all risks.
Postpartum & future
- 6-12 week postpartum OGTT (or HbA1c).
- Annual HbA1c thereafter.
- 50% lifetime T2DM risk — preventable with weight, exercise, diet.
- 50-70% GDM recurrence next pregnancy — early OGTT at 16-18 wk.
Breastfeeding with GDM
Recommended — reduces maternal T2DM risk and baby’s future diabetes / obesity risk.
Challenges: delayed lactogenesis; lower supply in some. Support early: skin-to-skin within first hour; frequent feeding; lactation consultant; antenatal hand-expressing of colostrum from 36-37 weeks if advised.
Different scenarios — GDM testing
Scenario 1: BMI 32, first pregnancy, OGTT booked 26 weeks
Standard pathway. Drink + 2 blood draws. Results 1-2 days. If positive, dietitian + glucose monitoring.
Scenario 2: Previous GDM, planning second pregnancy
Preconception HbA1c. Early OGTT 16-18 wk. Likely recurrence (~50-70%); ready to engage with care quickly.
Scenario 3: South Asian heritage, OGTT positive, fasting 5.8
GDM. Diet + exercise trial 2 weeks; if glucose targets not met, metformin. Growth scans. Plan 38-40 wk delivery.
Scenario 4: Severe NVP, can’t face glucose drink
Alternatives: CGM for 1-2 weeks; HbA1c blood test; home monitoring with finger-prick. Discuss with team.
Scenario 5: OGTT negative but big baby on 32-week scan
Repeat OGTT or random glucose. Sometimes GDM develops after 24-28 week test. Consider CGM. Manage as if GDM if growth scan concerns.
Care guidance — OGTT
- Fast overnight 10-12h before; water OK.
- Don’t restrict carbs in days leading up.
- Drink slowly over 5 min.
- Stay at clinic for 2h after drink.
- One value positive = GDM diagnosed.
- Engage early with dietitian + diabetes nurse.
- 4x daily glucose monitoring if GDM.
- Growth scans 28, 32, 36 wk.
- Antenatal colostrum harvesting from 36-37 wk if advised.
- Postpartum OGTT 6-12 wk.
- Annual HbA1c long-term.
Sources
- NICE NG3. Diabetes in pregnancy: management.
- WHO. Diagnostic criteria and classification of hyperglycaemia in pregnancy.
- IADPSG Consensus Panel. International Association of Diabetes and Pregnancy Study Groups recommendations.
- ACOG Practice Bulletin 190. Gestational diabetes mellitus.
- NHS. Gestational diabetes overview.
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Frequently asked questions
What is the OGTT and why do I need one?
ORAL GLUCOSE TOLERANCE TEST. Screens for GESTATIONAL DIABETES (GDM) — high blood sugar in pregnancy. AFFECTS ~1 IN 6 PREGNANCIES UK; rising globally. PROCEDURE: fast overnight (10-12 hours, water OK); blood drawn fasting; drink very sweet 75g glucose drink; blood drawn at 1 hour + 2 hours. ANY ONE result above threshold = GDM diagnosed. NHS UK: offers at 24-28 weeks for women with risk factors (and 16-18 weeks if previous GDM). PRIVATE: ~£100-200. NOT optional in most pathways if at risk.
What are the OGTT thresholds?
UK / WHO / IADPSG / NICE NG3 (one-step 75g): FASTING ≥5.6 mmol/L (101 mg/dL); 1 HOUR ≥10.0 mmol/L (180 mg/dL); 2 HOURS ≥7.8 mmol/L (140 mg/dL). ANY ONE = GDM. US ACOG (two-step) different. NUMBERS vary slightly by country. RESULTS arrive in 1-7 days depending on lab/system. SOME women diagnosed by single fasting reading; others only by post-drink values. ALL ARE TREATED similarly.
Who needs an OGTT in pregnancy?
RISK FACTORS (UK NICE NG3): (1) BMI ≥30; (2) Previous baby ≥4.5 kg; (3) Previous GDM in any pregnancy; (4) Family history of diabetes (parent / sibling); (5) Ethnicity at higher risk (South Asian, Middle Eastern, Black, Hispanic); (6) Previous unexplained stillbirth. UNIVERSAL screening: some countries / private; one risk factor = NHS offers OGTT 24-28 weeks; previous GDM = offered earlier at 16-18 weeks. IF NEGATIVE EARLY, repeat at 24-28 weeks. SOME experts argue for universal screening — missed cases without risk factors are common.
What does positive OGTT mean for my pregnancy?
GDM DIAGNOSIS. MANAGED with: (1) DIETITIAN referral — low GI foods, reduced refined sugars, smaller frequent meals; (2) HOME GLUCOSE MONITORING 4x/day (fasting + 1 hr post-meals); (3) METFORMIN if diet alone not controlling (now NHS first-line second-step); (4) INSULIN if metformin not enough; (5) GROWTH SCANS at 28, 32, 36 weeks; (6) DELIVERY plan: aim for 39-40 weeks (NICE NG3 evidence-based), C-section if EFW ≥4.5 kg; (7) NEONATAL hypoglycaemia checks for baby. POSTPARTUM: 6-12 week OGTT, annual HbA1c (50% lifetime T2DM risk).
Will I need insulin?
MAYBE. CONTROL HIERARCHY: (1) DIET + EXERCISE — works for ~30%; (2) METFORMIN (low-dose, gradually increased) — works for further ~30%; (3) INSULIN (rapid-acting at meals + long-acting at bedtime) — needed by ~30-40%. INSULIN NEEDS: increase through 3rd trimester as placenta hormones rise; PEAK ~34-36 weeks; rapid drop after delivery. INJECTIONS: subcutaneous (under skin, not into muscle); educated to self-inject; partner can help. MOST WOMEN with insulin stop it immediately after delivery (placenta removed = insulin resistance gone).
What can I eat with gestational diabetes?
LOW GI (low glycaemic index) foods — release sugar slowly. EAT: protein with every meal (eggs, fish, meat, beans); vegetables (especially green leafy); whole grains (oats, brown rice, quinoa, sourdough); berries and lower-sugar fruit (apples, pears); nuts; greek yoghurt; cheese; healthy fats. AVOID / LIMIT: white bread, white rice, sugary cereals, juice, sweets, soft drinks, fruit smoothies, pastries. EAT SMALLER PORTIONS more often (every 3 hours). KEY: protein + fibre + healthy fat with every carb. DIETITIAN essential for individual plan.
Can I drink the glucose drink test if I have severe morning sickness?
DIFFICULT. The drink is very sweet (75g sugar dissolved in water — like 6+ teaspoons sugar). MANY find nauseating. STRATEGIES: (1) very cold helps; (2) sip slowly over 5 minutes (within 5-15 min window); (3) bring ginger / mint to suck after; (4) bucket nearby — vomit invalidates result. IF UNABLE: alternatives — HBA1C blood test (less accurate, 5.7-6.4% suggests GDM); RANDOM glucose; CONTINUOUS GLUCOSE MONITORING (CGM) for 1-2 weeks. DISCUSS with team if true intolerance.
What happens at GDM antenatal clinic?
DEDICATED MULTI-DISCIPLINARY GDM clinic (some trusts) or joint obstetric + diabetic clinic. APPOINTMENTS: every 1-4 weeks (frequency depends on control). INVOLVES: (1) OBSTETRICIAN — pregnancy progression, growth scans; (2) DIABETIC SPECIALIST NURSE — glucose readings, medication adjustments; (3) DIETITIAN — food advice; (4) MIDWIFE — usual antenatal care; (5) FETAL MEDICINE if growth concerns; (6) NEONATAL alerts (baby will have glucose checks post-delivery). HOME MONITORING: 4x/day glucose readings recorded; reviewed at appointments.
Will my baby have problems?
MOST babies born to well-controlled GDM mothers: HEALTHY. POORLY CONTROLLED GDM risks: (1) BIG BABY (macrosomia / LGA) — birth trauma, shoulder dystocia; (2) NEONATAL HYPOGLYCAEMIA — baby's pancreas was producing extra insulin in utero; (3) JAUNDICE; (4) RESPIRATORY DISTRESS; (5) HIGHER chance of needing NICU briefly. LONG-TERM (mothers + babies): increased risk of obesity, T2DM, metabolic syndrome in later life. GOOD CONTROL minimises risks. POSTPARTUM: baby's heel-prick glucose monitoring first 24 hours; encourage early breastfeeding for blood sugar stability.
Will I have GDM in next pregnancy?
HIGHLY LIKELY ~50-70% recurrence. PRECONCEPTION CARE: HbA1c check 6-12 months before trying; LIFESTYLE optimisation; folic acid 5 mg if BMI ≥30. EARLY GDM SCREEN in next pregnancy (16-18 weeks vs 24-28 weeks usually). LIFESTYLE between pregnancies: weight management, regular exercise, balanced diet — can reduce recurrence and T2DM risk substantially. LONG-TERM: 50% lifetime risk T2DM — annual HbA1c monitoring; aspirin if other PE factors; metabolic screening.
Does GDM affect breastfeeding?
USUALLY positive. BREASTFEEDING reduces: maternal T2DM risk; baby's diabetes/obesity risk in later life. CHALLENGES: delayed lactogenesis (milk coming in slightly delayed) — common in GDM; lower supply in some. SUPPORT EARLY: skin-to-skin within first hour; frequent feeding (every 2-3 hours including night); LACTATION CONSULTANT support; hand-expressing colostrum antenatally (from 36-37 weeks, if recommended) helps emergency feeding. STORE colostrum in syringes for first hours. GLUCOSE MONITORING for baby first 24 hours — early feeding helps stabilise.
What about birth plans with GDM?
DELIVERY usually 38-40 weeks (NICE NG3): aim 38-39 if good control; 39-40 if excellent control; earlier if growth concerns. C-SECTION DISCUSSED if EFW ≥4.5 kg. INDUCTION: common at 38-39 wk if not in spontaneous labour. INTRAPARTUM monitoring: continuous CTG; insulin sliding scale if needed; tight glucose targets (4-7 mmol/L). NEONATAL TEAM alerted; baby checks blood sugar at 2, 4 hours post-feed. SKIN-TO-SKIN + immediate feeding reduces hypo risk. EPIDURAL fine; water birth + GDM possible if control good and no insulin.
Can I refuse the OGTT?
YES — screening is OPTIONAL. SOME refuse because: (1) prefer not to know unless symptoms; (2) horrible taste of drink; (3) feel low-risk. ALTERNATIVES: HOME GLUCOSE MONITORING 1-2 weeks; HBA1C blood test (less sensitive); CONTINUOUS GLUCOSE MONITOR (CGM); waiting for clinical signs (big baby, glucose in urine). RISK of refusing: missing GDM → poorly controlled → larger baby → birth complications, neonatal hypoglycaemia. INFORMED CHOICE — discuss with midwife. ANY ONE positive test on monitoring should prompt review.
What about the dawn phenomenon?
DAWN PHENOMENON: early morning glucose elevation due to growth hormones. COMMON in GDM and Type 1/2 diabetes. WORSENS later pregnancy. STRATEGIES: bedtime snack with protein + complex carb (toast + peanut butter, yoghurt + nuts); evening insulin timing adjustment; avoid late large carb-heavy meal; check 3 am glucose if persistent. NIGHTTIME hypos (low glucose) more common in insulin users — bedtime carb essential. IMPACT: morning fasting glucose hard to control; speak with diabetic specialist nurse if persistent.
Will I get diabetes after baby?
HIGHER RISK. 50% LIFETIME RISK of TYPE 2 DIABETES after GDM (vs ~10% general population). HALF develop T2DM within 10 years. PREVENTION: maintain healthy weight; regular exercise (150 min/week moderate intensity); balanced diet; metformin if continued risk. SCREENING: 6-12 WEEK postpartum OGTT or HbA1c; ANNUAL HbA1c thereafter — NHS / NICE recommend. NEXT PREGNANCY: 50% recurrence; pre-pregnancy HbA1c. NOT INEVITABLE — many women never develop T2DM with active prevention.
How does this relate to other calculators on BumpBites?
Companion: /calculators/gdm-risk for risk assessment before testing; /calculators/gct-50g for two-step screening; /calculators/insulin-pregnancy for managing if GDM; /calculators/aspirin-pe-prevention; /calculators/fetal-weight for growth scans; /calculators/pcos-pregnancy (overlap risk factor); /calculators/pregnancy-bmi; /calculators/pregnancy-nutrition.