Epidemiology

GDM is a growing health concern in many parts of the world - it occurs in 2-5% pregnancies but figures vary considerably depending upon the criteria used. Only 10% of these patients would be diabetic if it wasn't for the pregnancy effects on glucose (ie 10% were diabetic but coincidentally diagnosed when pregnant).

Risk factors^[1][6]

GDM is more likely with:

• Increasing age
• Certain ethnic groups (Asian, African Americans, Hispanic/Latino Americans and Pima Indians)
• High BMI before pregnancy (three-fold risk for obese women compared to non-obese women)
• Smoking - it doubles the risk of GDM^[7]
• Change in weight between pregnancies - an inter-pregnancy gain of more than three units (of BMI) doubles the risk of GDM ^[8]
• Short interval between pregnancies
• Previous unexplained stillbirth
• Previous macrosomia
• Family history of type 2 diabetes or GDM - more relevant in nulliparous than parous women^[9]

Protective factors

• Physical activity, especially vigorous activity before pregnancy and at least light-to-moderate activity during pregnancy may reduce risk for abnormal glucose tolerance.^[10]
• Bariatric surgery reduces the future risk of developing GDM in obese women. ^[11]

Diagnosis

Screening

GDM is usually asymptomatic but has serious consequences that can be reduced by treatment,^[12] making it a candidate for screening.

NICE recommends:^[13]

• Where a woman has had previous GDM, she should be offered early self-monitoring of blood glucose or a two-hour 75 g OGTT at 16-18 weeks, followed by a repeat OGTT at 28 weeks if the first test is normal
• Other risk factors should prompt an OGTT at 24-28 weeks.
• Screening for GDM should not be undertaken using fasting plasma glucose, random blood glucose, glucose challenge test or urinalysis for glucose. Whilst we routinely dip urine at every antenatal visit, the detection of glucosuria is not a sufficiently good screening tool for GDM and may delay diagnosis, adversely affecting outcome.

Timing of diagnosis:

• GDM is rare before 20 weeks,^[14] and early diagnosis of GDM is associated with poor maternal and fetal outcome.^[15] This probably reflects earlier presentation of the more severe cases or pre-existing but unrecognised diabetes. Those with pregestational diabetes are at much higher risk of having babies with a serious birth defect than those who develop diabetes in the second and third trimester.^[1]
• Glucose tolerance changes with the duration of the pregnancy so that the gestation at which the diagnosis was made should be recorded and, if made in the third trimester, there should be some caution about the clinical implications of impaired glucose tolerance (IGT).^[14]

Management^[14]

Treatment of GDM and good glycaemic control reduces serious perinatal morbidity and may also improve the woman's health-related quality of life.^[12]

It is often controversial as to when GDM requires treatment. A recent study showed that the treatment (dietary intervention, self-monitoring of blood glucose, and insulin therapy, if deemed necessary) of mild GDM (defined as positive OGTT but fasting glucose<5.3 mmol/L) did reduce the risks of fetal overgrowth, shoulder dystocia, Caesarean delivery, and hypertensive disorders but not a composite primary outcome (based upon stillbirth or perinatal death and neonatal complications, including hyperbilirubinaemia, hypoglycaemia, hyperinsulinaemia, and birth trauma).^[16]

Antenatal

Women should receive routine antenatal care but in addition:

• Monitoring:
  • Women are usually taught how to monitor their blood sugars, at least daily (dependent on treatment) and educated regarding individualised target levels. In general, women should aim for fasting blood glucose of between 3.5 and 5.9 mmol/L and one-hour postprandial blood glucose below 7.8 mmol/L.^[13]
  • Urinalysis (looking for protein) and blood pressure checks should be performed at antenatal appointments, as women with GDM are at higher risk of hypertensive disorders.
  • Ultrasound should be performed monthly to assess fetal growth and amniotic fluid volume from 28 weeks. Macrosomia is usually taken as dimensions above the 95th centile for that period of gestation. Measurement of abdominal circumference of the baby can exclude macrosomia and reduce the need for insulin without impairing outcome.^[12]
• Non-drug treatment:^[13]
  • Body weight - weight loss should be aimed for with women with a BMI >27.
  • Diet - all women with GDM should receive dietary advice from a specialist dietician - food choices should reflect the nutritional demands of pregnancy and concentrate on the need for micronutrient-rich food.
  • Physical activity - encourage at least 30 minutes' physical activity per day, sufficient to induce slight breathlessness.
• Medication:^[13]
  • Hypoglycaemic therapy should be considered where:
    • Lifestyle adjustments fail to maintain blood glucose targets over 1-2 weeks
    • Ultrasound at diagnosis shows incipient fetal macrosomia (abdominal circumference >70th percentile).
  • Where hypoglycaemic therapy is required:
    • It should be tailored to individual needs.
    • Regular insulin, the rapid-acting insulin analogues aspart and lispro, and/or the oral hypoglycaemic agents metformin and glibenclamide may be considered.
  • Current evidence base does not suggest a significant difference between the use of oral hypoglycaemic agents and insulin in women with GDM, both for maternal glycaemic control, rates of Caesarean delivery and neonatal outcomes (such as infant birth weight, neonatal hypoglycaemia, and congenital anomalies).^[17]
  • Insulins are widely used in pregnancy and generally accepted as safe. There is some evidence of improved control with a basal bolus regimen. Mothers receiving insulin therapy and those in regular contact with them should be instructed on the recognition, management and treatment of hypoglycaemia.
  • Metformin is widely used in the UK for the management of diabetes in pregnancy and lactation. It appears both effective and safe, despite SPC advice that it should be avoided when planning pregnancy and during pregnancy. Similarly, glibenclamide's SPC advises that it is contra-indicated in pregnancy, despite evidence for its effectiveness and safety. Thus, informed consent should be obtained and documented prior to the use of both these drugs in GDM.

Intrapartum

• GDM is a risk factor for Caesarean delivery (due to the risk of macrosomia and dystocia). The place and mode of delivery should depend on the state of health of mother and fetus.
• NICE guidelines suggest offering women with GDM (as for those with established diabetes) induction of labour (or Caesarean section) from 38 weeks where the baby has grown normally. Where women opt to wait for spontaneous labour, they should be offered weekly tests for fetal well-being from 38 weeks.
• During delivery, blood glucose should be measured hourly (half hourly if a general anaesthetic is used) and maintained at between 4 and 7 mmol/L.
• Babies should be encouraged to feed as soon after birth as possible (within 30 minutes) and every 2-3 hours thereafter. Blood glucose should be tested within 2-4 hours of birth or if there are signs of hypoglycaemia. Infants should not be discharged within 24 hours of birth, until feeding well and maintaining their blood glucose levels (>2 mmol/L pre-feed).

Postpartum^[18]

• Insulin and other hypoglycaemics are usually stopped immediately following delivery.
• The mother's blood glucose should be checked prior to discharge to the community. Fasting blood glucose is usually checked 6 weeks postpartum. Where this is diagnostic doubt, an OGTT should be pursued at this stage as if diagnosing diabetes mellitus in a patient who had never been pregnant.^[19]
• Counselling regarding the increased risk of diabetes and recurrence of gestational diabetes in subsequent pregnancies. There is some evidence that progression to type 2 diabetes can be reduced by regular physical activity and avoiding obesity.
• Women who have had GDM should be advised to have annual fasting blood glucose tests.

Complications^[12]

The risk of perinatal mortality is not increased but there are

• Increased perinatal risks of:
  • Macrosomia
  • Shoulder dystocia
  • Birth injuries such as bone fractures and nerve palsies
  • Hypoglycaemia
• Long-term adverse health outcomes in infants born to mothers with gestational diabetes include:
  • Sustained impairment of glucose tolerance
  • Subsequent obesity (although not when adjusted for size)
  • Impaired intellectual achievement

For the women themselves, gestational diabetes is a strong risk factor for diabetes and metabolic syndrome.^[20] Undoubtedly, interventions (diet, lifestyle, medication) that could help prevent or delay the onset of type 2 diabetes in these women would have an important effect on their long-term health. However the effectiveness of such interventions remains largely untested in clinical trials to date.^[5]

Prognosis

• The perinatal risks to mother and baby are similar to those with known diabetes, mainly relating to the problems of a large baby.
• Most women will apparently recover after the pregnancy but with a 2 in 3 chance of recurrence in a future pregnancy.^[6]
• The highest fasting glucose level during pregnancy, followed by the severity of glucose intolerance, and earlier gestational diabetes are the best predictors for postpartum diabetes.^[21] IGT in the first few months after delivery is associated with a high risk of diabetes in the near future.^[22]
• Repeated pregnancy, particularly close together, does appear to increase the risk of developing diabetes (whereas combined oral contraceptives and hormone replacement therapy do not).^[23]
• Children whose mothers had GDM are more likely to be obese^[24] but this does not necessarily imply a genetic or intrauterine effect. The parents are more likely to be overweight too and a shared environment may lead to similar attitudes and behaviours towards food.

Advice after gestational diabetes

Having had GDM, you are at significantly increased risk of developing diabetes and so:

• Achieve and maintain a satisfactory BMI.
• Take regular exercise.
• Do not smoke.
• Do not have pregnancies in rapid succession.
• Make sure you attend your six-week postpartum check and have a fasting blood glucose test taken.
• Have annual fasting blood glucose tests.

See separate article Prevention of Type 2 Diabetes.