Gestational diabetes mellitus

Gestational diabetes is defined as any degree of glucose intolerance with onset or first recognition during pregnancy. GDM often precedes a diagnosis of T2DM, occasionally it is a new presentation of T1DM. Management includes a combination of diet, metformin and insulin therapy.

OGTT

At present there is little consensus about precisely which OGTT protocol to use and what cut-offs should be applied when diagnosing GDM.

One option is to use WHO criteria for diagnosis of gestational diabetes mellitus; these are a fasting plasma glucose >7 mmol/L, or a two hour plasma glucose >7.7 mmol/L in a 75g OGTT performed at 24-28 weeks gestation. At present, NICE recommends using the WHO criteria.

Other criteria are available and the IADPSG's (International Association of Diabetes and Pregnancy Study Groups) recommendations on diagnosis and management of hyperglycaemia in pregnancy are the newest and amongst the most authoritative. These recomend using a 75 g OGTT with diagnostic cut offs of 5.1, 10.0 and 8.5 mmol/L plasma glucose at fasting, 1, and 2 hours respectively. If any one of these 3 levels are elevated, a diagnosis is made. However, adoption of these guidelines is likely to lead to a large increase in the number of cases of GDM being diagnosed, raising issues around capacity in many health systems. These issues have been reviewed recently in a RCOG scientific advisory group opinion paper.

ECG

Not normally necessary but may be performed if there is suspicion of pre-existing ischaemic heart disease.

HbA1c

This is a useful baseline.

Auto antibodies

These should only be tested if there is some suspicion that the patient has T1DM presenting during pregnancy. 

C-peptide levels

This is rarely a useful test in GDM but may be contributory if there is some suspicion that the patient actually has T1DM.

USS

Obstetric USS to detect foetal macrosomia or IUGR and/or polyhydramnios are important investigations, not just for the obstetrician but also for the diabetologist as they will inform decisions on whether and when to initiate hypoglycaemic therapy in a proportion of cases. 

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