Explaining Gestational Diabetes

Definition

Gestational diabetes is a type of diabetes that occurs only during pregnancy and usually resolves after giving birth.

Causes

Gestational diabetes is primarily caused by hormonal changes in pregnancy that lead to increased insulin resistance, meaning the body’s cells become less responsive to insulin. During pregnancy, the pancreas typically compensates for insulin resistance by producing more insulin. However, in some women, the pancreas cannot keep up with the increased demand for insulin, leading to elevated blood sugar levels.

Risk factors also play significant roles in the development of gestational diabetes, including:

• • Obesity (BMI >30 kg/m²)
  • Age: Women >25 are at greater risk
  • Ethnic Origin: Higher prevalence in Afro-Caribbean, Middle Eastern, and South Asian women.
  • Family history: First-degree relatives with diabetes
  • Previous gestational diabetes
  • Previous macrosomic baby (≥ 4.5kg)

Clinical Presentation

Gestational diabetes often does not cause symptoms. However, some people develop symptoms, which may be mild, such as increased thirst, more frequent urination, fatigue, or nausea.

Diagnosis

NICE guidelines suggest all pregnant women with risk factors for gestational diabetes, which are identified at booking, should be screened at 24-28 weeks with an oral glucose tolerance test (OGTT). Another reason to screen for gestational diabetes is glycosuria, using either 2+ on one occasion or 1+ on two occasions. 

The exception to this is women with previous gestational diabetes, who should either self-monitor their blood glucose or have a OGTT soon after the booking clinic (the first midwife appointment during pregnancy that ideally takes before 10 weeks gestation), with a repeat test at 24 weeks if this is normal.

OGTT:

• An OGTT is performed in the morning after an overnight fast. The test involves the patient drinking a 75g glucose drink first thing in the morning. Blood glucose levels are measured before consuming the sugar drink (fasting) and 2 hours after drinking it.
• The normal OGTT results are:
  • Fasting: <5.6 mmol/l
  • 2 hours: <7.8 mmol/l
  • A simple way to remember the thresholds for diagnosing gestational diabetes are “5-6-7-8”. If the fasting glucose is ≥5.6 mmol/L or the 2-hour glucose is ≥7.8 mmol/L, a diagnosis of gestational diabetes can be made. 

Management

A combination of strategies is used in the management of gestational diabetes:

• Patients with gestational diabetes are managed in combined diabetes and antenatal clinic, with input from a dietitian.
• They require careful explanation about the condition, including the importance of keeping sugar levels within the normal range to prevent complications. They also need to be taught how to monitor and track their blood sugar levels.
• It is recommended that women with gestational diabetes take a fasting blood glucose level each day and test blood glucose levels 1 hour after every meal.
• They require 4 weekly ultrasound scans from 28-36 weeks gestation to monitor foetal growth and amniotic volume.
• The initial management suggested by NICE depend on the patient’s fasting glucose at diagnosis:
  • Fasting glucose <7 mmol/l 🡪 trial of diet and exercise for 1-2 weeks
    • If targets are not met, metformin should be commenced
    • If targets are not met following the addition of metformin, insulin should be added 
  • Fasting glucose >7 mmol/l 🡪 insulin should be started immediately +/- metformin
  • Fasting glucose >6 mmol/l plus macrosomia (or other complications) 🡪 insulin should be started immediately +/- metformin
• Advice on dietary changes should be given by a dietitian.
• Glibenclamide (a sulfonylurea) is suggested by NICE as an option for women who cannot tolerate metformin or for women are not well controlled on metformin but decline taking insulin. 
  • NB – Glibenclamide is not licensed for this indication, and it is advised that the prescriber takes full responsibility for this decision and obtains informed written consent.

• Delivery:
  • Women with gestational diabetes can give birth up to 40+6
  • All medications should be stopped at delivery
  • A sliding-scale insulin regime is considered during labour in cases of poorly controlled blood glucose levels
• Postpartum:
  • Blood glucose levels should be checked in hospital to ensure they are normal.
  • A few weeks after giving birth, women who have had gestational diabetes should be offered a test to ensure they do not have type 2 diabetes or pre-diabetes.
  • Ideally, this should be a fasting plasma glucose test conducted between 6 and13 weeks postpartum
  • If the test is delayed beyond 13 weeks, a fasting plasma glucose test or HbA1c test can be carried out
  • It is also important to inform the mother that she is at high risk of developing gestational diabetes in future pregnancies 

Complications

There are several complications of gestational diabetes for both the mother and foetus:

Maternal Complications:

• Pre-eclampsia: Higher blood glucose levels can increase the risk of developing pre-eclampsia
• A large for dates foetus and macrosomia: This can lead to serious problems during delivery, including injuries to the foetus, genital tract lacerations, and an increased risk of C-section. The most serious concern is shoulder dystocia, which is when the baby’s head is born but one of the shoulders becomes stuck behind the mother’s pubic bone, delaying the birth of the baby’s body.
• Recurring Gestational Diabetes: There is a higher risk of having gestational diabetes in subsequent pregnancies.
• Future Diabetes: There is a higher risk of developing type 2 diabetes later in life.

Foetal Complications:

• Neonatal Hypoglycaemia: This occurs because gestational diabetes results in the baby having increased amounts of glucose in their blood, which stimulates its pancreas to secrete more insulin. However, after birth, the baby no longer receives large amounts of glucose from its mother, but still has hyperinsulinemia. This increases the risk of hypoglycaemia. Postnatal care involves checking the baby’s blood glucose regularly and frequent feeds to keep the baby’s blood glucose >2mmol/L. If the baby’s blood glucose falls <2mmol/L, IV dextrose or nasogastric feeding may be required to correct it.
• Pre-term Birth: This may lead to developmental issues and respiratory distress syndrome due to immature lungs.
• Obesity and Type 2 Diabetes: There is an increased risk of developing obesity and type 2 diabetes later in life.
• Jaundice: Babies are more likely to develop jaundice due to hyperbilirubinemia.

Pre-conception:

• It is important to advise women with existing diabetes to avoid unplanned pregnancies and aim to achieve the target HbA1c in planned pregnancies. This is because there are several well-established risks (e.g. first-trimester miscarriage, premature delivery, and congenital malformation) of poor glycaemic control at conception.
• Women should take 5mg folic acid from preconception until 12 weeks gestation.
• Women with existing type 1 and type 2 diabetes should aim for the same target blood glucose levels as with gestational diabetes
• Women with type 2 diabetes are managed using metformin and insulin. Any other oral diabetic medications should be stopped.

Diabetic Retinopathy screening

• Should be performed shortly after booking and at 28 weeks gestation

Delivery:

• NICE advise a planned delivery between 37 and 38+6 weeks
• A sliding-scale insulin regime is considered during labour for women with type 1 diabetes or women with 2 diabetes with poorly controlled glucose levels

Postpartum:

• Women with existing diabetes should lower their insulin doses and be cautious of hypoglycaemia in the postnatal period. This is because insulin sensitivity increases after birth and with breastfeeding. 

Blood glucose targets for gestational diabetes and pre-existing diabetes are the same. The NICE blood glucose targets for fasting, 1-hour post-meal, 2 hours post-meal and avoiding levels are:

• Fasting: 5.3 mmol/l
• 1-hour post-meal: 7.8 mmol/l
• 2 hours post-meal: 6.4 mmol/l
• Avoiding levels: ≤4 mmol/l 

HbA1c is a poor marker of glycaemic control in the second and third trimesters, which is when most cases of gestational diabetes are diagnosed. The OGTT is therefore preferred.