How is gestational diabetes diagnosed

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Diagnosis is carried out in one or two phases. The first phase is a 50 g oral glucose tolerance test at the 24-28th week of gestation, which the pregnant woman receives regardless of prior food ingestion. Venous plasma glucose values above 140mg/dl (7.8mmol/L) one hour after glucose ingestion are considered abnormal, although some authors suggest a lower level (130 mg/dl [7.2 mmol/L]). When a cut-off point of 140 mg/dl (7.8 mmol/L) is used, around 80 percent of gestational diabetic women are detected, whereas with 130 mg/dl (7.2 mmol/L) as cut-off, around 90 percent are detected. At the same time, however, the false positive rate is increased, i.e. women wrongly considered to have gestational diabetes. This particular glucose tolerance test is not accepted by all, though it is easy and cheap, as it includes only one blood glucose measurement, regardless of food intake.

If an abnormal result is found in the 50 g oral glucose tolerance test, a second oral glucose tolerance test follows, either with 100 g glucose (three hours) or with 75 g glucose (two hours). There is no unanimous agreement whether an increased intake of carbohydrates for three days should precede the test, although this has been proposed. Diagnostic criteria for gestational diabetes, depending on the method used, are shown in Table 10.1.

It should be noted that there is disagreement over whether to screen all pregnant women without exceptions for gestational diabetes or only those at high risk. The same disagreement applies over the methods and diagnostic criteria. Most authors, however, insist on screening all pregnancies without exceptions, given the fact that hyperglycaemia management during gestation protects from a series of complications, whereas during selection some risk factors may be missed.

CASE STUDY 1

A 35 year old healthy woman, with a history of an unexplained miscarriage six months ago, wishes to become pregnant again. Is there a chance that this woman will develop gestational diabetes and how will the diagnosis be made?

Table 10.1. Oral glucose tolerance test during pregnancy

100 g oral glucose tolerance test (adapted from O'Sullivan, 1964)

For diagnosing gestational diabetes mellitus, at least two abnormal values are required:

Fasting plasma glucose > 95 mg/dl (5.3 mmol/L)

75 g oral glucose tolerance test

American Diabetes Association Criteria

At least two values higher than the following: Fasting plasma glucose

1 hour

2 hours

World Health Organization Criteria

Fasting plasma glucose or

2 hours

The chance that a woman will develop gestational diabetes mellitus is large when at least one of the following applies:

1. family history of DM, especially in first degree relatives;

2. body weight before pregnancy larger than ideal body weight, at least by 10 percent;

3. age more than 25 years;

4. history of impaired glucose tolerance;

5. a member of an ethnic group with high prevalence of diabetes (black race, etc.);

6. a previous unexplained perinatal interruption of pregnancy or intrauterine death or birth of a neonate with congenital abnormalities or with a body weight of more than 4 kg (8.82 lb);

7. multiparous (more than four pregnancies).

Some authors have added more risk factors.

Regardless of the above situations that require greater vigilance from the clinician and possibly indicate the need for screening from the first clinic visit, screening for gestational diabetes should be performed, according to most authors, in all apparently normal pregnancies at the 24-28th week of gestation.

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