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Brody S, Harris RP, Whitener BL, et al. Screening for Gestational Diabetes Mellitus [Internet]. Rockville (MD): Agency for Healthcare Research and Quality (US); 2003 Feb. (Systematic Evidence Reviews, No. 26.)

  • This publication is provided for historical reference only and the information may be out of date.

This publication is provided for historical reference only and the information may be out of date.

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Screening for Gestational Diabetes Mellitus [Internet].

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4Discussion

Benefits and Harms of Screening and Treatment

Maternal and fetal morbidity increase with increasing levels of maternal glycemia. Screening and intensive treatment for gestational diabetes mellitus (GDM) aim to reduce this morbidity. Various screening strategies can detect women with different levels of hyperglycemia, but the cutpoint at which health outcomes begin to deteriorate to a clinically important degree, given today's obstetric care, is uncertain.

The magnitude of any benefit of intensive treatment at various levels of glycemia associated with GDM is uncertain, but it is likely to be small among the many women with lower glycemic levels. The magnitude of the harms and costs of screening and intensive treatment is also uncertain, but potentially substantial.

We have no direct evidence about the health outcomes of screening compared with no screening. No controlled trial has examined the benefit of either universal or selective screening compared to routine care without screening. In addition, little information is available from well-conducted and analyzed randomized controlled trials (RCTs) that have examined the benefit of universal versus selective screening.

The direct evidence about the health outcomes of intensive treatment of women with GDM at various levels of maternal glycemia is limited by the small number of studies, the small number of participants, the lack of masking of obstetrical care, the lack of control for important confounders, and the lack of emphasis on health outcomes rather than intermediate outcomes. Observational studies provide some indirect evidence that screening may reduce macrosomia, an intermediate outcome, but the extent to which this provides a further health benefit is unclear.

For women with GDM who have higher levels of glycemia, intensive treatment likely reduces macrosomia. The extent to which this translates into a reduction in birth trauma is uncertain but is substantially less than the reduction in macrosomia. For women with GDM with lower levels of glycemia, the existing evidence does not show that dietary therapy improves important clinical outcomes compared with no diet therapy. About 70% or more of all women with GDM are in this group.

By making various assumptions, we can calculate illustrative values for the number needed to screen (NNS) -- i.e., the number of women needed to screen to prevent 1 case of brachial plexus injury (Table 7). If we take Case 1, for example, and assume that 4% of pregnant women have GDM, 1 that 30% of them will have a high enough glycemic level to require insulin, 130 and that, among these women, the macrosomia rate is reduced to 9% (i.e., the degree seen in the most positive study 106 ), then the NNS to prevent 1 brachial plexus injury is about 8,900. At least 80% of brachial plexus injuries resolve within the first year of life.1,58,66,67 If we make more generous assumptions that 6% GDM has a prevalence of 6% (essentially, a high-risk group) and that 50% of women with GDM are treated with insulin (Case 3), then the NNS is about 3600. If, to Case 3, we add the further assumptions that infants less than 4,000 g also benefit -- the best-case scenario shown in the footnote to Table 7 -- the NNS becomes 3,300.

Table 7. Number Needed to Screen (NNS) for Gestational Diabetes Mellitus (GDM) to Prevent 1 Case of Brachial Plexus Injury.

Table

Table 7. Number Needed to Screen (NNS) for Gestational Diabetes Mellitus (GDM) to Prevent 1 Case of Brachial Plexus Injury.

One potential benefit of detecting women with GDM is the knowledge that they have a higher risk for developing type 2 diabetes. The extent to which this information can lead to a health benefit for the women is uncertain. 120 A recent review of screening for type 2 diabetes found that screening primarily benefits people with increased cardiovascular risk factors. 215 Thus, women with hypertension might well be screened, but the benefit of detecting diabetes earlier among young women with few cardiovascular risk factors is not clear.

GDM may also have long-term implications for the offspring, such as increased risk of childhood obesity, glucose intolerance, or neuropsychological disturbances. Current data are limited and mixed about these issues. Data are insufficient to show that routine screening will significantly influence these outcomes.

The evidence concerning the harms and costs of screening and intensive treatment is even more limited than the evidence about benefits, but several harms are of concern. Many women may suffer anxiety of uncertain duration because of a false-positive screening test. Labeling women with GDM as having an increased risk of future GDM and type 2 diabetes may have psychological implications. Detection of GDM may increase the probability of cesarean delivery; multiple antenatal tests may increase the probability of a false-positive test leading to unnecessary procedures. Costs may be increased with little health benefit for many women, especially those many women with lower levels of hyperglycemia.

Future Research

It is difficult to see how the issue of screening for GDM versus no screening can be clarified without RCTs that mask obstetrical care and examine health outcomes, not simply intermediate outcomes. Ideally, the RCT would compare screening with no screening among women without pregestational diabetes. Some clinicians might consider such a study unethical, but we think the level of uncertainty in this situation gives adequate justification for at least consideration of such a study, if not directly moving ahead.

If, however, an RCT of screening per se cannot be mounted, another approach might be a series of RCTs focused on treatment. First-round RCTs should begin with women diagnosed with GDM who have lower levels of hyperglycemia; they should compare intensive glycemic control and antepartum surveillance with usual non-GDM obstetrical care. If intensive treatment does not demonstrate improved health outcomes among these women, then a second round of RCTs ought to investigate the effects of therapy among women with higher levels of hyperglycemia should be studied. These studies should also monitor and report harms and costs associated with screening and intensive treatment.

In summary, the issue of screening for GDM remains a contentious one. The reason for this controversy is largely a lack of high-quality research addressing the central issues of both screening and therapy. Only good research can end the controversy and tell us how to best serve women and their infants.

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