Gestational Diabetes Mellitus (GDM)
Summary
Gestational Diabetes Mellitus (GDM) is defined as any degree of glucose intolerance with onset or first recognition during pregnancy. It results from the inability of the maternal pancreas to overcome the physiological insulin resistance of pregnancy (driven by Human Placental Lactogen). GDM is associated with significant maternal and fetal risks, including macrosomia, shoulder dystocia, pre-eclampsia, and neonatal hypoglycaemia. Diagnosis is typically via Oral Glucose Tolerance Test (OGTT) at 24-28 weeks. Management involves intensive glycaemic control through diet, metformin, and/or insulin to minimise adverse outcomes. [1,2]
Key Facts
- Prevalence: 5-15% of pregnancies (rising with obesity rates).
- Diagnosis (NICE): Fasting plasma glucose ≥5.6 mmol/L OR 2-hour plasma glucose ≥7.8 mmol/L.
- Main Fetal Risk: Macrosomia (due to fetal hyperinsulinaemia) → Shoulder Dystocia.
- Main Neonatal Risk: Hypoglycaemia (fetal pancreas hyperactive after cord cutting).
- Maternal Future Risk: 50% lifetime risk of developing Type 2 Diabetes.
- Management: Diet/Exercise (70% managed), Metformin, Insulin.
Clinical Pearls
The "Fetal Insulin Hypothesis": Maternal glucose crosses the placenta freely; maternal insulin does NOT. High maternal glucose → High fetal glucose → Fetal Pancreas makes High Fetal Insulin. Insulin is a growth factor → Macrosomia.
Neonatal Hypoglycaemia: When the cord is cut, the glucose supply stops abruptly, but the baby still has high insulin levels. This causes a crash in blood sugar. Essential to feed early and monitor.
Steroids Spike Usage: If giving antenatal corticosteroids (for preterm lung maturity) to a GDM mother, expect massive hyperglycaemia. Always use a variable rate insulin infusion (VRII) or increased insulin protocol during steroid administration.
Post-Natal Check: Don't forget the mother! She needs a Fasting Glucose (or HbA1c) at 6-13 weeks post-partum to check the diabetes has resolved.
Incidence and Demographics
- Prevalence: Affects ~5-15% of pregnancies in the UK/USA.
- Ethnicity: Higher prevalence in South Asian, Black Caribbean, and Middle Eastern populations.
Risk Factors for Screening (NICE Criteria)
Any one of the following warrants an OGTT:
- BMI >30 kg/m².
- Previous Macrosomic Baby (≥4.5 kg).
- Previous GDM.
- Family History of diabetes (1st degree relative).
- High-Risk Ethnicity (South Asian, Black, Middle Eastern).
Timing of Screening
- Risk Factors Present: OGTT at 24-28 weeks.
- Previous GDM: Offer early self-monitoring or OGTT at booking (12-14 weeks) AND at 24-28 weeks if initial test normal.
- Glycosuria: 2+ glucose on one occasion or 1+ on two occasions warrants testing.
Physiological Insulin Resistance
- Pregnancy is a diabetogenic state.
- Key Hormones: Human Placental Lactogen (hPL), Cortisol, Progesterone, Prolactin.
- These hormones increase insulin resistance to shunt glucose to the fetus.
- Resistance peaks in the third trimester (24-28 weeks onwards).
Pathogenesis of GDM
- Normal pregnancy: Maternal beta-cells undergo hyperplasia to produce 2-3x more insulin.
- GDM: Maternal beta-cells fail to compensate for the increased resistance.
- Result: Maternal hyperglycaemia.
Fetal Pathophysiology (Pederson Hypothesis)
- Maternal Hyperglycaemia.
- Transplacental Glucose Transfer.
- Fetal Hyperglycaemia.
- Fetal Beta-cell Hyperplasia (Fetal Hyperinsulinaemia).
- Fetal Effects:
- Macrosomia: Insulin is an anabolic growth factor (fat/organ deposition).
- Polycythaemia: High oxygen demand → increased erythropoietin.
- Surfactant deficiency: Insulin delays maturation of type II pneumocytes (RDS risk).
Maternal Symptoms
Antenatal Signs
Red Flags
- Reduced Fetal Movements: Risk of stillbirth.
- Rapid abdominal growth: Polyhydramnios.
- Visual disturbances/Headache: Pre-eclampsia co-existence.
Antenatal Checks
- SFH Measurement: Assess for acceleration.
- BP: Check for pre-eclampsia.
- Oedema: Generalised.
Diagnostic Criteria (NICE NG3)
75g Oral Glucose Tolerance Test (OGTT)
| Time point | Threshold (NICE) | Threshold (WHO/IADPSG) |
|---|---|---|
| Fasting | ≥ 5.6 mmol/L | ≥ 5.1 mmol/L |
| 2-Hour | ≥ 7.8 mmol/L | ≥ 8.5 mmol/L (and 1hr ≥10.0) |
Note: NICE criteria (UK) differ slightly from WHO criteria.
Monitoring (Once Diagnosed)
- Capillary Blood Glucose (CBG):
- Fasting (Pre-prandial).
- 1-hour Post-prandial.
- Ultrasound Scans:
- Growth scans every 4 weeks (28, 32, 36 weeks).
- Assess EFW (Estimated Fetal Weight) and Liquor (AFI).
HbA1c
- Not used for GDM diagnosis (reduced reliability in pregnancy due to RBC turnover).
- Used at booking to identify pre-existing T2DM.
Management Algorithm
DIAGNOSIS OF GDM
(Fasting ≥5.6 OR 2h ≥7.8)
↓
Evaluate Fasting Glucose
↓
┌─────────────────┴─────────────────┐
↓ ↓
FASTING less than 7.0 mmol/L FASTING ≥7.0 mmol/L
↓ (Possible overt DM)
TRIAL OF DIET/EXERCISE ↓
(1-2 weeks) IMMEDIATE INSULIN
↓
Monitors Targets:
Fasting less than 5.3
1h Post less than 7.8
↓
Targets Met? ──→ NO ──→ ADD METFORMIN
↓ (or Insulin)
YES ↓
CONTINUE DIET Still High?
ADD INSULIN
1. Diet and Lifestyle (First Line)
- Low Glycaemic Index (GI) diet.
- Portion control.
- Moderate exercise (30 mins walk/day).
- Effective in ~70% of women.
2. Pharmacotherapy
Indications: Fasting ≥7.0 at diagnosis, or failure of diet therapy after 1-2 weeks.
-
Metformin:
- First line oral agent.
- Safety: Crosses placenta but good evidence for safety.
- Mechanism: Improves insulin sensitivity.
- Side effects: GI upset.
-
Insulin:
- Gold Standard for tight control.
- Indications: Contraindication to Metformin, maximal Metformin dose, or Fasting ≥7.0.
- Regimens: Basal (NPH) or Basal-Bolus.
- Does NOT cross placenta.
-
Glibenclamide:
- Rarely used now (only if Metformin/Insulin declined/not tolerated). High risk of neonatal hypoglycaemia.
3. Intrapartum Management
- Monitoring: Hourly CBG during labour. Target 4.0-7.0 mmol/L.
- Insulin/Dextrose: If CBG >7.0 or usually on insulin, may need GKI (Glucose-Potassium-Insulin) sliding scale.
- Pre-term steroids: Increase insulin dramatically if steroids given.
4. Obstetric Management
- Timing of Birth:
- Diet controlled: By 40+6 weeks.
- Medically treated: By 38+0 - 39+0 weeks.
- Mode of Birth:
- GDM is not a contraindication to vaginal birth.
- Offer Caesarean if EFW >4.5kg (risk of shoulder dystocia).
Maternal
- Pre-eclampsia: 2-4x risk.
- Polyhydramnios: Risk of unstable lie, cord prolapse.
- Birthing Injury: Perineal tear (3rd/4th degree) due to macrosomia.
- Caesarean Section: Increased rate.
- Future T2DM: 50% risk within 10 years.
Fetal / Neonatal
- Macrosomia: Birth weight >4.0kg or >90th centile. Organomegaly (hepatomegaly, cardiomegaly).
- Shoulder Dystocia: Medical emergency.
- Neonatal Hypoglycaemia: Seizures/brain injury if untreated.
- RDS: Delayed lung maturity.
- Polycythaemia / Jaundice.
- Stillbirth: Risk increased if control poor.
Post-Natal
- Resolution: GDM usually resolves immediately after placental delivery. Stop medications.
- Follow-up Screening:
- Fasting Plasma Glucose (or HbA1c) at 6-13 weeks post-partum.
- Annual HbA1c thereafter.
Long Term
- Child: Increased risk of childhood obesity and metabolic syndrome ("fetal programming").
- Mother: High risk of recurrence in next pregnancy and T2DM.
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| NG3 | NICE (2015/2020) | Diagnostic thresholds 5.6/7.8; Metformin first line. |
| GDM Practice Bulletin | ACOG | Different screening (2-step: 50g challenge -> 100g OGTT). |
| Metformin in GDM | RCOG / NICE | Acceptable as safe alternative to insulin. |
Landmark Studies
1. HAPO Study (2008) [3]
- Question: Is mild hyperglycaemia harmful?
- Result: Linear relationship between maternal glucose and adverse outcomes (macrosomia, C-section, neonatal hypoglycaemia) even below diabetic range.
- Impact: Led to lowering of diagnostic thresholds (IADPSG criteria).
2. ACHOIS Trial (2005) [4]
- Question: Does treating GDM improve outcomes?
- Result: Treatment reduced serious perinatal complications (death, dystocia, nerve palsy) from 4% to 1%.
- Impact: Confirmed value of screening and treating GDM.
What is Gestational Diabetes?
It is a type of diabetes that affects women during pregnancy. The placenta produces hormones that help the baby grow, but these hormones also block your own insulin from working properly. Usually, your body makes extra insulin to cope, but in GDM, it can't keep up, and your blood sugar rises.
Will my baby have diabetes?
No, your baby does not have diabetes. However, your high sugar crosses to the baby, making the baby produce too much of its own insulin. This acts like a "growth hormone," making the baby grow very large (macrosomia).
Risks to the Baby
- Growing too big: Makes birth difficult (shoulder getting stuck).
- Low sugar after birth: Because the baby is used to high sugar from you, when the cord is cut, their sugar can drop dangerously low. We need to feed them quickly.
Treatment
- Diet: Most women can control it by cutting out sugary foods and eating healthy carbohydrates.
- Medication: Some women need tablets (Metformin) or Insulin injections to keep sugar safe for the baby.
After Birth
- The diabetes usually goes away as soon as the baby is born.
- However, you are at higher risk of developing Type 2 Diabetes later in life, so keep a healthy diet and get checked annually.
Primary Sources
- NICE Guideline NG3. Diabetes in pregnancy: management from preconception to the postnatal period. 2015 (Updated 2020).
- American Diabetes Association. Management of Diabetes in Pregnancy. Diabetes Care. 2023;46(Suppl 1):S254-S266.
- HAPO Study Cooperative Research Group. Hyperglycemia and adverse pregnancy outcomes. N Engl J Med. 2008;358:1991-2002. PMID: 18463375.
- Crowther CA, et al. Effect of treatment of gestational diabetes mellitus on pregnancy outcomes (ACHOIS). N Engl J Med. 2005;352:2477-2486. PMID: 15951574.
- Farrar D, et al. Different strategies for diagnosing gestational diabetes to improve maternal and infant health. Cochrane Database Syst Rev. 2017;CD007122.
Common Exam Questions
- Obstetrics: "A pregnant woman at 26 weeks has BMI 35. What is the screening test?"
- Answer: OGTT (Oral Glucose Tolerance Test). Not HbA1c.
- Medicine: "Diagnostic criteria for GDM (NICE)?"
- Answer: Fasting ≥5.6 OR 2-hour ≥7.8.
- Neonatology: "Newborn of diabetic mother is jittery. Diagnosis?"
- Answer: Neonatal Hypoglycaemia. Check blood glucose.
- Pharmacology: "Patient on Metformin for GDM. Can she breastfeed on it?"
- Answer: Yes, Metformin is safe during lactation.
Viva Points
- Shoulder Dystocia: Explain mechanism. (Fat deposition on fetal shoulders/trunk > head size).
- Post-natal: Why check at 6-13 weeks? To ensure HPL effects have washed out and distinguish GDM from underlying T2DM.
- Pathophysiology: Role of HPL (Human Placental Lactogen) as an insulin antagonist.
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.