Gestational diabetes mellitus (GDM) is glucose intolerance during pregnancy. Although most cases resolve after the birth of the baby, there is an increased risk of developing type 2 diabetes in the future.
GDM is the most common medical condition complicating pregnancy and its incidence is rising in the UK. Currently, incidence of GDM in the UK is around 3.5%.
Patients with GDM have at least a sevenfold increased risk of developing diabetes later in life. It is an important condition to diagnose and treat, because treatment has been shown to reduce the incidence of obesity and diabetes in the future.
GDM is associated with an increased risk of induction of labour, assisted delivery, caesarean section, macrosomia, neonatal hypoglycaemia, neonatal jaundice, respiratory distress, stillbirth, and brain, spinal cord and cardiac defects.
Compared with pregnant women with normal glucose tolerance, women with GDM have slightly greater insulin resistance in late pregnancy and lower insulin secretion for their degree of resistance.
GDM is due to maternal insulin insufficiency and resistance due to pancreatic beta-cell dysfunction. This is thought to be due to autoimmune beta-cell dysfunction, genetic abnormalities and beta-cell dysfunction associated with chronic insulin resistance.1
The maternal hyperglycaemia that occurs leads to fetal hyperinsulinaemia and increased fetal growth.
Insulin secretion decreases in relation to chronic insulin resistance over time, causing progressive hyperglycaemia and type 2 diabetes after pregnancy.
Currently, different criteria are used for the diagnosis of GDM (see box, above).
However, the International Association of Diabetes and Pregnancy Study Groups (IADPSG) is trying to achieve international consensus for screening and diagnosis.2 It also recommends a 75g oral glucose tolerance test (OGTT) for all women not known to be diabetic at 24-28 weeks' gestation.
The IADPSG predicts that using its diagnostic criteria, which have been adopted in the SIGN guideline on management of diabetes,3 will increase the per pregnancy incidence of GDM to more than 16%.
However, NICE recommends that women with any one of the risk factors listed in the box (below right) and marked with an asterisk should be offered an OGTT at 24-28 weeks.4 Around 30-50% of pregnant women in the UK have at least one of these risk factors.
NICE also recommends that women who have had GDM in a previous pregnancy should be offered early self-monitoring of blood glucose or an OGTT at 16-18 weeks and again at 28 weeks (if results are normal).
Screening for GDM using fasting plasma glucose, random blood glucose, glucose challenge test and urinalysis for glucose should not be undertaken.
|Risk factors for GDM|
Lifestyle advice (including dietary modification) is recommended for all cases. Most women will not need medication for their GDM.
Glibenclamide and metformin are effective oral treatments. Both appear to be safe when taken during pregnancy. Insulin is needed in 20-30% of women who are initially treated with oral hypoglycaemics. Women receiving insulin should be given information on the recognition, management and treatment of hypoglycaemia.
The Maternal-Fetal Medicines Unit Network trial of treatment of mild GDM found significant reductions in mean birthweight and caesarean section rate with treatment that improved glycaemic control.7
A study in Australia established that treatment of GDM with insulin improved both pregnancy and serious perinatal outcomes.8 However, there seems to be no significant difference in outcome between oral hypoglycaemic drugs and insulin in women with GDM.9
The lowest risk of complications (including birthweight >4kg, prematurity, pre-eclampsia and neonatal hypoglycaemia) has been shown to occur with fasting glucose ≤4.9mmol/L.5
One study demonstrated that if all overweight and obese women (BMI ≥25kg/m2) had a GDM risk equal to that of normal weight women, nearly half of all GDM cases could be prevented. The authors suggest that public health efforts to reduce pre-pregnancy BMI by promoting physical activity and healthy eating among women of reproductive age should be intensified.10
- Dr Newson is a GP in the West Midlands
1. Metzger BE, Buchanan TA, Coustan DR et al. Summary and recommendations of the Fifth International Workshop-Conference on Gestational Diabetes Mellitus. Diabetes Care 2007; 30 Suppl 2: S251-60.
2. IADPSG Consensus Panel. IADPSG recommendations on the diagnosis and classification of hyperglycemia in pregnancy. Diabetes Care 2010; 33(3): 676-82.
3. SIGN. Management of diabetes. Guideline No 116; March 2010.
4. NICE. Diabetes in pregnancy. CG63. NICE, London, 2008.
5. Hedderson MM, Gunderson EP, Ferrara A. Gestational weight gain and risk of gestational diabetes mellitus. Obstet Gynecol 2010; 115(3): 597-604.
6. Roos N, Kieler H, Sahlin L et al. Risk of adverse pregnancy outcomes in women with polycystic ovary syndrome. BMJ 2011; 343: d6309.
7. Landon MB, Spong CY, Thom E et al. A multicenter, randomized trial of treatment for mild gestational diabetes. N Engl J Med 2009; 361: 1339-48.
8. Crowther CA, Hiller JE, Moss JR et al. Effect of treatment of gestational diabetes mellitus on pregnancy outcomes. N Engl J Med 2005; 352: 2477-86.
9. Nicholson W, Bolen S, Witkop CT et al. Benefits and risks of oral diabetes agents compared with insulin in women with gestational diabetes. Obstet Gynecol 2009; 113(1): 193-205.
10. Kim SY, England L, Wilson HG et al. Percentage of gestational diabetes mellitus attributable to overweight and obesity. Am J Public Health 2010; 100(6): 1047-52.