Gestational diabetes refers to diabetes triggered by pregnancy. It is caused by reduced insulin sensitivity during pregnancy, and resolves after birth.
The most significant immediate complication of gestational diabetes is a large for dates fetus and macrosomia. This has implications for birth, mainly posing a risk of shoulder dystocia. Longer-term, women are at higher risk of developing type 2 diabetes after pregnancy.
Anyone with risk factors should be screened with an oral glucose tolerance test at 24 – 28 weeks gestation. Women with previous gestational diabetes also have an OGTT soon after the booking clinic.
Risk Factors
The NICE guidelines (2015) list the risk factors that warrant testing for gestational diabetes:
- Previous gestational diabetes
- Previous macrosomic baby (≥ 4.5kg)
- BMI > 30
- Ethnic origin (black Caribbean, Middle Eastern and South Asian)
- Family history of diabetes (first-degree relative)
Oral Glucose Tolerance Test
The screening test of choice for gestational diabetes is an oral glucose tolerance test (OGTT). An OGTT is used in patients with risk factors for gestational diabetes, and also when there are features that suggest gestational diabetes:
- Large for dates fetus
- Polyhydramnios (increased amniotic fluid)
- Glucose on urine dipstick
An OGTT should be performed in the morning after a fast (they can drink plain water). The patient drinks a 75g glucose drink at the start of the test. The blood sugar level is measured before the sugar drink (fasting) and then at 2 hours.
Normal results are:
- Fasting: < 5.6 mmol/l
- At 2 hours: < 7.8 mmol/l
Results higher than these values are used to diagnose gestational diabetes.
TOM TIP: It is really easy to remember the cutoff for gestational diabetes as simply 5 – 6 – 7 – 8.
Management
Patients with gestational diabetes are managed in joint diabetes and antenatal clinics, with input from a dietician. Women need careful explanation about the condition, and to learn how to monitor and track their blood sugar levels. They need four weekly ultrasound scans to monitor the fetal growth and amniotic fluid volume from 28 to 36 weeks gestation.
The initial management suggested by the NICE guidelines (2015) is:
- Fasting glucose less than 7 mmol/l: trial of diet and exercise for 1-2 weeks, followed by metformin, then insulin
- Fasting glucose above 7 mmol/l: start insulin ± metformin
- Fasting glucose above 6 mmol/l plus macrosomia (or other complications): start insulin ± metformin
Glibenclamide (a sulfonylurea) is suggested as an option for women who decline insulin or cannot tolerate metformin.
Women need to monitor their blood sugar levels several times a day. The NICE (2015) target 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 of 4 mmol/l or below
Pre-Existing Diabetes
Before becoming pregnant, women with existing diabetes should aim for good glucose control. They 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 insulin levels as with gestational diabetes. Women with type 2 diabetes are managed using metformin and insulin, and other oral diabetic medications should be stopped.
Retinopathy screening should be performed shortly after booking and at 28 weeks gestation. This involves referral to an ophthalmologist to check for diabetic retinopathy. Diabetes carries a risk of rapid progression of retinopathy, and interventions may be required.
NICE (2015) advise a planned delivery between 37 and 38 + 6 weeks for women with pre-existing diabetes. (Women with gestational diabetes can give birth up to 40 + 6).
A sliding-scale insulin regime is considered during labour for women with type 1 diabetes. A dextrose and insulin infusion is titrated to blood sugar levels, according to the local protocol. This is also considered for women with poorly controlled blood sugars with gestational or type 2 diabetes.
TOM TIP: It is worth remembering the importance of retinopathy screening during pregnancy for women with existing diabetes. This is an exam favourite, and will score you extra points with your seniors if you mention it in the antenatal clinic.
Postnatal Care
Diabetes improves immediately after birth. Women with gestational diabetes can stop their diabetic medications immediately after birth. They need follow up to test their fasting glucose after at least six weeks.
Women with existing diabetes should lower their insulin doses and be wary of hypoglycaemia in the postnatal period. The insulin sensitivity will increase after birth and with breastfeeding.
Babies of mothers with diabetes are at risk of:
- Neonatal hypoglycaemia
- Polycythaemia (raised haemoglobin)
- Jaundice (raised bilirubin)
- Congenital heart disease
- Cardiomyopathy
Babies need close monitoring for neonatal hypoglycaemia, with regular blood glucose checks and frequent feeds. The aim is to maintain their blood sugar above 2 mmol/l, and if it falls below this, they may need IV dextrose of nasogastric feeding.
TOM TIP: If you remember two complications of gestational diabetes, remember macrosomia and neonatal hypoglycaemia. Babies become accustomed to a large supply of glucose during the pregnancy, and after birth they struggle to maintain the supply they are used to with oral feeding alone.
Last updated September 2020