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 June 2, 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.