Gestational Diabetes

Gestational diabetes mellitus (GDM) is defined as any degree of glucose intolerance with its onset (or first recognition) during pregnancy.¹ There is little consensus about tighter definitions or diagnostic criteria, but WHO's definition is the most widely used.

Only 10% of diabetes in pregnancy is pregestational (ie previously diagnosed Type 1 or 2 diabetes) so most is gestational. GDM occurs in 2-5% pregnancies but figures vary considerably depending upon criteria used. Some do not recover after pregnancy and it is possible that they had undiagnosed type 1 or 2 diabetes, especially if sugar levels are very high.

Risk factors⁴

GDM is more likely with:

• Increasing age
• Certain ethnic groups (African Americans, Hispanic/Latino Americans and American Indians)³
• High BMI before pregnancy
• Smoking doubles the risk of gestational diabetes⁵
• Change in weight between pregnancies - an inter-pregnancy gain of more than 3 units (of BMI) doubles the risk of GDM ⁶
• Short interval between pregnancies
• Previous unexplained stillbirth
• Previous macrosomia
• Family history of DM- more relevant in nullips than parous women⁷

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.⁸

Screening

GDM is usually asymptomatic but has serious consequences that can be reduced by treatment⁹, making it a candidate for screening. Contenders for screening tests are glucose challenge (mini-GTT) and fasting glucose test (venous or capillary¹⁰ blood specimens). However, currently in the UK, NICE does not recommend routine antenatal screening due to lack of evidence but do maintain that healthcare providers "remain alert to signs or symptoms of conditions which affect the health of the mother and fetus, such as.. diabetes". ¹¹ Thus clinicians must decide on whom they should focus their efforts and usually a risk factor based scheme is used to determine pregnancies that should receive further testing for GDM. This method will only detect about 50% of women with gestational diabetes.¹²

Urinalysis

Urine should be checked for glucose at every antenatal visit. Renal blood flow and glomerular filtration rate (GFR) rise in pregnancy with the result that the renal threshold for glycosuria is reduced. Some feel that the practice of waiting for repeatedly positive urine tests for glucose prior to obtaining a blood sample delays diagnosis and may adversely affect outcome.²

Glucose tolerance tests

A 75 g oral glucose tolerance test (OGTT) should be conducted if the blood glucose exceeds 5.5 mmol/l at 2 hours or more after food, or exceeds 7 mmol/l within 2 hours of food. Glucose tolerance tests may be unreliable especially as gastric emptying is delayed in pregnancy. If the OGTT is performed at or before 16 weeks gestation, a negative result does not necessarily exclude future problems and if the results are borderline the test should be repeated between 32 and 34 weeks.¹³

Early diagnosis of GDM is associated with poor maternal and fetal outcome.¹⁴ Rather than suggesting that management is counterproductive, this probably means that the more severe cases present earlier.

Treatment of gestational diabetes reduces serious perinatal morbidity and may also improve the woman's health-related quality of life.⁹

Antenatal

Therapy is usually guided by fetal growth and glycaemic monitoring:

• If there is gross abnormality of blood sugar this must be corrected as a matter of urgency.
• Monitoring:
  • Women are usually taught how to monitor their blood sugars, at least daily (dependant on treatment) and educated regarding target levels.
  • 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 is performed to assess for macrosomia, normally from about 28 weeks with frequency dependant on local policy. 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.⁹
• Non-drug treatment:
  • 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. Active weight reduction is not thought to be appropriate due to risk of compromising maternal and fetal health.
  • If there is no macrosomia and after dietary advice the blood glucose levels before and after meals are normal, treat as normal.
  • If, after dietary advice, fasting glucose levels exceed 6mmol/l and 2 hours postprandial the figure is over 7mmol/l, then medical therapy is required - usually involving insulin.
  • Physical activity - encourage at least 30 minutes physical activity per day, sufficient to induce slight breathlessness.
• Medication:
  • If there is not macrosomia but glucose levels are in the diabetic range, intensive therapy is required as in diabetes. No insulins are currently licensed for pregnancy but most are widely used and accepted to be 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.
  • The role of oral hypoglycaemic agents in pregnancy is being reassessed- a paper from the USA described the use of glyburide (glibenclamide in UK) in GDM with some benefit but possibly an increased risk of pre-eclampsia.¹⁵ Previously, there have been concerns about risk of congenital malformations associated with sulphonylureas in pregnancy.

GDM is a risk factor for caesarian 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.

Postpartum¹⁶

• Insulin can usually be stopped immediately following delivery.
• Usual practice is to check fasting blood glucose post-partum. Final reassessment of maternal glycemic status should be performed at least 6 weeks after delivery - 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.¹
• 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.

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
  • Hypoglycemia
• 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.¹⁷

1. The perinatal risks to mother and baby are similar to those with known diabetes, mainly relating to the problems of a large baby.
2. Most women will apparently recover after the pregnancy but with a 2 in 3 chance of recurrence in a future pregnancy.⁴
3. 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.¹⁸ Impaired glucose tolerance in the first few months after delivery is associated with a high risk of diabetes in the near future.¹⁹
4. Repeated pregnancy, particularly close together, does appear to increase the risk of developing diabetes (whereas combined oral contraceptives and hormone replacement therapy do not).²⁰
5. Children whose mothers had GDM are more likely to be obese²¹ 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. .