Background

ifT"), In normal pregnancy, placental production of diabetogenic hormones, such as growth hormone, corticotrophin-releasing hormone, placental lactogen and progesterone, induces maternal insulin resistance and compensatory hyperinsulinaemia. In some women, pancreatic insulin production cannot compensate for this insulin resistance and carbohydrate intolerance develops. Gestational diabetes mellitus (GDM) is defined as carbohydrate intolerance that begins or is first diagnosed in pregnancy. Type 1 diabetes, type 2 diabetes and monogenic diabetes can also present in pregnancy and care must be taken to differentiate these from true GDM, which should resolve after delivery.

GDM is associated with significantly increased feto-maternal morbidity, with increased rates of pre-eclampsia, polyhydramnios and macrosomia. Feto-maternal injuries during birth and subsequent development of diabetes in the mother are also increased. Though the associations are clear, there is significant disagreement about screening and treatment strategies. This is largely because, until very recently, there have been no data linking screening for GDM to improved feto-maternal outcome. Because GDM is frequently associated with other adverse features, such as maternal obesity, it has not been clear whether intervening to treat modest hyperglycaemia is beneficial. Indeed, excessive lowering of maternal glycaemia has been hypothesized to cause fetal growth retardation. Birth injury consequent upon macrosomia is also rarely permanent. It has been estimated that 450 cae-sarean sections would need to be performed to prevent one permanent neurological disability from shoulder dystocia.

There are a number of screening strategies.

1 Some centres advocate a stringent practice of universal screening for GDM.

2 The American Diabetes Association1 currently recommends using an assessment of clinical risk factors at the first antenatal visit to support selective screening for GDM as shown in Table 25.1. One-step or two-step screening/diagnostic strategies are then supported:

• The one-step approach may be more cost effective in women from ethnic groups at higher risk of GDM. A 100 gram 3-hour, or 75 gram 2-hour, oral glucose tolerance test (oGTT) is performed with samples obtained at baseline and then hourly. GDM is diagnosed if two or more glucose values are abnormal (thresholds are 5.3, 10.0,8.6 and 7.8 mmol/l at 0, +1, +2 and +3 hours, respectively);

• The two-step strategy uses an initial 50 gram oral glucose challenge test (threshold of 7.8 mmol/l at +1 hour) to identify women at risk of GDM, who are then offered an oGTT as above.

3 Another approach is to use risk factors to selectively screen women at high risk for gestational diabetes using a 75 gram oGTT at 28 weeks' gestation. GDM is diagnosed if fasting glucose is ^7.0 mmol/l or 2-hour glucose is ^7.8 mmol/l.

A recent review concluded that a lack of high-quality evidence meant that it was impossible to determine the effect that any of these strategies might have on neonatal and maternal outcomes.2

Once GDM is diagnosed, there are three aspects to treatment: dietary modification, self-monitoring of blood glucose levels and insulin administration. Dietary modification stresses selection of a varied diet containing complex rather than simple carbohydrates. Some limitation of carbohydrate and fat intake is also usual—particularly carbohydrate at breakfast when insulin resistance is greatest. Emphasis is placed on weight maintenance rather than weight reduction.

Current technologies mean that self-monitoring of blood glucose is relatively simple. The suggested practice is to advise pre- and post-prandial monitoring, with a pre-prandial target of 4-6 mmol/l and a 2-hour post-prandial target of 6-8 mmol/l. Post-prandial monitoring is recommended because one study reported that a 1-hour

j American Diabetes Association screening protocol for GDM

Risk of GDM

Risk factors

Action

Marked obesity

Personal history of GDM Glycosuria

Strong family history of diabetes

Screen at first visit. If normal, rescreen between 24 and 28 weeks' gestation

Average risk

Neither high nor low risk

Screen between 24 and 28 weeks' gestation

Age <25 years

Weight normal before pregnancy

Member of an ethnic group with a low prevalence of GDM

No known diabetes in first-degree relatives

No history of abnormal glucose tolerance

No history of poor obstetric outcome

No screening necessary

Low Risk

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