Informed Decision-Making Issues
Should you agree to be screened?
(Screening consists of having blood drawn and blood glucose measured usually an hour after drinking a concentrated glucose solution. Women screening positive go on to a definitive test, the Oral Glucose Tolerance Test. The OGTT usually involves having blood drawn after an overnight fast and then one, two, and three hours after drinking the sugar solution.)
Pros: Some women may be undiagnosed diabetics or have been tipped into true diabetes by the extra demands of pregnancy (about 1 in 1,000 pregnant women) (3). Screening will reveal this. Developing gestational diabetes in pregnancy indicates higher risk for developing true diabetes later in life. This may provide greater motivation to lose weight and exercise regularly, which may reduce this risk (13).
Cons: Neither the OGTT nor the screening test are reliable tests in that they give different results when repeated in the same person (8,24). In addition, blood glucose values rise as pregnancy advances, but no adjustments are made for this. This means you could “fail” a test in week 28 that you would have “passed” had you taken it in week 24 (17). The various thresholds used to diagnose GD are purely arbitrary (11,28-29,40,43). None of them correlate with the appearance of or a marked increase in complications. Studies fail to show that treatment reduces adverse outcomes such as overlarge babies (16). However, being identified as a gestational diabetic greatly increases the chance of having a cesarean simply because of the diagnosis, not because of problems such as overlarge baby (5,21,37,42). For these reasons, several organizational bodies have opposed GD testing. A Guide to Effective Care in Pregnancy and Childbirth, the bible of evidence-based care, relegates screening, diet, and diet plus insulin to “Forms of Care Unlikely to be Beneficial (16).” The American College of Obstetricians and Gynecologists says no data support the benefits of screening (1). The U.S. Preventative Services Task Force and the Canadian Task Force on the Periodic Health Examination both conclude that there is insufficient evidence to justify universal GD screening (7,15).
Suggestion: Before agreeing to be screened, you should know how your doctor or midwife handles GD. You might consider being screened but refusing extra care and treatment except when test results indicate true diabetes. If you agree to screening, take the test as early in pregnancy as it is offered (46). Also, take it early in the day; sugar values tend to be lower than in the afternoon (2). If you are not told to fast, eat one to two hours before the test (44). Insist on using the World Health Organization or the National Diabetes Data Group criteria for the screening test and, if you get that far, the OGTT. They have higher thresholds than other testing schemes in common use. Setting lower thresholds greatly increases the number of women diagnosed as gestational diabetics without improving outcomes (39,43).