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.

Diet

Dietary management is the first line of management in all cases of GDM and for some will be all that is required to maintain satisfactory glycaemia throughout pregnancy.

Dietary management should be aimed at the avoidance of refined sugars and saturated animal fats and in women who are overweight or obese at conception should also aim for a caloric restriction of 25 Kcal/kg/day.

Dietetic referral is mandatory in all cases and regular dietetic review should be a routine part of ongoing care.

Exercise

Unrealistic targets should not be set. Any increase in routine physical activity will be beneficial.

For many people, half-an-hour walking each day on most days of the week will represent a significant increase in their physical activity and will have tangible benefits in terms of reduction in weight and increase in general well being.

Smoking cessation

The potential benefits of smoking cessation should be promoted very strongly and patients should be offered professional help with smoking cessation which may take the form of group support or one to one counselling.

Smoking cessation will also have huge benefits for foetal health.

Glycaemic targets

Pregnant women with GDM may need to test capillary blood sugar levels up to seven times per day, before and one hour after each meal and before bed time.

Ideal glycaemic targets are blood sugar 4-5.9 mmol/L pre-prandial and less than 8 mmol/L one hour post prandial.

These targets are difficult to consistently achieve and in some women will lead to an unacceptable incidence of hypoglycaemia, in which case individualised targets should be agreed with the patient.

Many centres will check HbA1c periodically during pregnancy but recent guidance in the UK from the NICE actually advises against routine use of HbA1c in the second and third trimester.

Oral hypoglycaemics

Sulphonylureas

The sulphonylureas were formerly considered to be contraindicated in GDM.

They are now being used in some centres, although they are not licensed for this indication.

Thiazoledinediones ('glitazones')

The glitazones are contraindicated in GDM.

Metformin

Although not licensed for this indication, metformin therapy is increasingly used in GDM. If it is to be used, it should be started after failure to achieve satisfactory glucose control after a trial of optimal dietary management.

Metformin should be started at a dose of 500mg twice a day and progressively titrated up to a maximum of 2-2.5 g/day in 500mg increments.

Even when dietary management is adequate on its own to achieve good glycaemic control at first, as the pregnancy progresses glycaemic targets will be exceeded and metformin may be required.

Metformin (or insulin) therapy should also be initiated even in the presence of apparently adequate glycaemia if the foetus is becoming macrosomic.

Insulin therapy

Insulin therapy is still the mainstay of treatment of GDM. If it is not possible to achieve adequate glycaemia with metformin alone, or if the patient is metformin intolerant, or if the foetus is becoming macrosomic despite optimal metformin therapy then insulin should be initiated.

At first this will often be in the form of prandial insulin only taken with the main meal of the day and titrated to achieve a one hour post prandial blood glucose of less than 8 mmol/L. As pregnancy progresses it will often be necessary to add additional doses of insulin with the other meals and ultimately to add a basal insulin to control pre-prandial blood sugar levels such that the patient is on a full basal-bolus regimen.

The prandial insulin can be in the form of soluble human insulin such as Humulin-S or may be a rapid-acting analogue such as Insulin Lyspro (Humalog). The basal insulin is usually a human-sequence isophane insulin such as Humulin-I. Long acting analogues such as insulin detemir (Levemir) are sometimes used.

An alternative to this stepwise progression to a basal bolus regimen is to commence insulin treatment directly with a twice daily biphasic insulin such as Humulin M3.

Intrapartum care

The capillary blood sugar level should be monitored hourly during labour and if it falls outside the 4-7 mmol/L range then intravenous insulin and glucose infusions should be started. These can normally be stopped shortly after delivery.

The blood sugar should continue to be monitored but will often fall back in to the normal range without additional treatment.

The patient should then be offered a fasting blood glucose test or an OGTT at the six week post natal check and a fasting blood glucose annually thereafter to test for development of T2DM.

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