Gestational diabetes mellitus (GDM) is a hyperglycaemic state that occurs during pregnancy. It occurs in about 4-5% of pregnancies. It most commonly occurs in the second or third trimester and poses risks to both the mother and baby.
Pathophysiology
The exact aetiology of the disease is not well understood. It is believed to do with circulating hormones in pregnancy that increase peripheral insulin resistance to ensure that sufficient glucose is available for the foetus. Some of these include oestrogen, cortisol, as well as human placental lactogen (hPL).
The placenta grows and produces more of these hormones which increases the risk of resistance and this is why it is more prevalent in later stages of pregnancy. ß-cells of the pancreas usually can accommodate and produce more insulin to match the increased demands. However, eventually the demand surpasses the production → GDM.
Circulating insulin passes through the placenta which causes cell growth (due to growth stimulatory actions of insulin). This leads to macrosomia. As maternal blood sugar levels are constantly raised, this also leads to pancreatic hyperplasia in the neonate to meet the insulin demands. However, after delivery when the baby no longer has high blood sugar levels, the insulin production surpasses the demand → nenonatal hypoglycaemia.
⚠️ Risk factors
- Previous gestational diabetes
- BMI >30
- Previous macrosomic baby
- Ethnic origin - black Caribbean, Middle Eastern and South Asian
- Family history of diabetes (1º relative)
- Maternal age >40
- Polycystic ovarian syndrome (PCOS)
😷 Presentation
The disease is usually asymptomatic.
Symptoms that may present are typical of diabetes, that being:
- Polyuria
- Polydipsia
- Fatigue
Signs indicative of GDM may include:
- Large for date foetus
- Polyhydramnios
- Glucose on urine dipstick
🔍 Screening and investigations
⭐️ NICE recommends a 75g 2-hour oral glucose tolerance test (OGTT) between 24-28 weeks gestation to high-risk women.
What qualifies a women as high risk?
One or more of the following risk factors:
- BMI >30kg/m2
- Previous baby weighing >4.5kg
- Family history of diabetes mellitus
- Family origin in an area of high prevalence of diabetes (black Caribbean, Middle Eastern and South Asian)
- Features that suggest GDM:
- Large for date foetus
- Polyhydramnios
- Glucose on urine dipstick
If a women has had a previous gestational diabetes, they should be offered early self-monitoring of blood glucose or an OGTT as soon as possible (at booking appointment). If the booking appointment OGTT is normal → repeat at 24-28 weeks.
OGTT is performed fasted in the morning. The baseline blood sugar levels are recorded and then the patient drinks 75g of a glucose drink and their blood sugar levels are recorded at 2 hours.
Let’s look at the diagnostic thresholds for diagnosing GDM:
- Fasting glucose - >5.6mmol/L is positive for GDM.
- 2-hour OGTT - >7.8mmol/L is also positive for GDM.
💡 Just remember 5 6 7 8.
- HbA1c - may be useful to identify if the diabetes was pre-existing or if the onset is gestational. It needs to be done at the booking appointment for all women with pre-existing diabetes. If a woman is diagnosed with gestational diabetes then the HbA1c needs to be done to assess if it was pre-existing.
- Urinalysis - identifies high-risk women.
🧰 Management
Antenatal care
- Newly diagnosed women need to be seen in a joint diabetic and antenatal clinic.
- Education of self-monitoring of blood glucose. Pregnant women should be advised to keep capillary glucose levels below the following ranges:
- Fasting - 5.3mmol/L
- 1 hour after meal - 7.8mmol/L
- 2 hour after meal - 6.4mmol/L
- Dietary advice - eating foods with low glycaemic index.
- Foetal growth monitoring - this should be done with ultrasound every 4 weeks from weeks 28-36.
- 🥇 Trial diet and exercise should be offered
- 🥈 If glucose targets are not met within 1-2 weeks → start metformin.
- 🥉 If glucose targets are still not met or metformin is contraindicated/poorly tolerated → add insulin (rapid-acting insulin).
- 🥇 Insulin - needs to be given immediately as rescue therapy to get it under control immediately.
- Insulin isophane - is recommended first-line by NICE. It is given once at night, or twice daily.
- Insulin glargine/insulin detemir - are also commonly used.
- 🥈 Metformin - 500mg OD metformin can be considered according to NICE, as an adjunct with insulin.
If fasting plasma glucose is 6-6.9mmol/L but there is evidence of macrosomia or polyhydramnios then follow the same protocol.
- Rapid-acting insulins (aspart and lispro) are favoured for diabetes in pregnancy.
- Women on insulin need to be advised on hypoglycaemia and advised to keep fast-acting glucose options available in case of hypoglycaemic events. Glucagon may be provided to women with type 1 diabetes mellitus to prevent hypoglycaemic attacks.
- Weight loss - if BMI >27kg/m2.
- Stop oral anti-hyperglycaemic agents (besides metformin) and replace them with insulin (sliding-scale insulin regime). Postnatally these should be reduced immediately
- Folic acid - 5mg/day until 12 weeks gestation.
- Anomaly scan - at 20 weeks. This includes cardiac anomalies.
- Glycaemic control
- Retinopathy screening - through ophthalmology referral performed at 28 weeks gestation.
Intrapartum care
- Type 1/2 diabetes and pregnant:
- No complications → induced labour or C-section between 37 weeks - 38 weeks + 6 days.
- Metabolic, maternal or foetal complications → before 37 weeks.
- Gestational diabetes:
- No complications → no later than 40 weeks + 6 days.
- Complications → before 40 weeks + 6 days.
Capillary plasma glucose readings should be taken hourly during labour with targets between 4-7mmol/L. If not maintained between this range then use IV insulin and IV dextrose to maintain this range.
Postnatal care
Pre-existing diabetes
- Reduce insulin.
- Advise women to have a snack/meal before or during feeds to prevent hypolycaemia.
Gestational diabetes
- Stop anti-hyperglycaemic agents if gestational diabetes. Me
- Test blood glucose levels prior to discharge to identify if hyperglycaemia is persisting.
- Offer lifestyle advice, plasma glucose test 6-13 weeks after birth, or HbA1c/fasting plasma glucose test after 13 weeks.
🚨 Complications
Maternal complications
- Gestational hypertension or pre-eclampsia
- Type 2 diabetes mellitus
- Recurrent GDM
- Non-elective C-section
Foetal complications
- Macrosomia - leads to subsequent complications such a non-elective C-section and shoulder dystocia.
- Neonatal hypoglycaemia - babies need to be monitored with regular blood glucose testing and frequent feeds. Blood glucose needs to be maintained >2mmol/L. If it drops below 2mmol/L then IV dextrose + nasogastric feeding should be done.
- Neonatal polycythaemia
- Neonatal jaundice
- Congenital heart defects and cardiomyopathy
- Diabetes mellitus, cardiovascular issues, obesity are also some notable long-term consequences.