In 1964 O'Sullivan and Mahan reported that pregnant women with glucose values at the upper end of the spectrum were more likely to develop diabetes later in life; the added stress of pregnancy revealed a woman's "predaibetic' status. Since diabetes was known to pose serious threats to the fetus, researchers extrapolated that subdiabetic levels of glucose intolerance during pregnancy might also do harm.
During the 1960s and 1970s doctors began studying the effects of glucose intolerance in pregnant women; however, the studies were poorly designed [and] thoroughly obscured the true risk of subdiabetic glucose intolerance in pregnancy. Results convinced researchers that they had discovered a serious problem, and in 1979 they convened the first of what became a series of exponentially larger international conferences.
Opening the first conference, one of the organizers suggested that pregnancy be viewed as a "tissue culture experience." Given the preconceived notions of the researchers, the confused state of the research and a metaphor that reduced women to incubators supplying potentially faulty growth medium, it should come as no surprise that by the end of the second conference, gestational diabetes (GD) was established as a new disease. It was officially defined as: "carbohydate intolerance of variable severity with onset or first recognition during the present pregnancy--irrespective of whether or not insulin is used for treatment or the condition persists after pregnancy. [It includes] the possibility that the glucose intolerance may have antedated the pregnancy." (Second International Workshop-Conference 1985)
Thus, women with blood glucose values in roughly the upper 3% for pregnant women have come to be defined as diabetics, although the situation is different from either type of true diabetes. The only problem GD shares with Type I and Type II diabetes is that chronic hyperglycemia can overfeed the fetus, resulting in macrosomia . Even here, other factors-race, age, parity and especially maternal weight far outweigh glucose intolerance in determining the baby's weight.
The conference definition of GD confuses more than it enlightens because it jumbles together various levels of severity. This is similar to claiming that everyone with a cough and fever has pneumonia. The confusion was deliberate. The conferees considered using the term "glucose intolerance of pregnancy" but decided on "diabetes" to make sure insurance companies would pay for high-risk management and women themselves would take the condition seriously.
The 1985 conference recommended-and the 1990 conference reaffirmed-that all pregnant women be screened for GD between 24 and 28 weeks by a 50 g glucose drink and that those with values of 140 mg/dl or above be given a diagnostic 100-g oral glucose tolerance test (OGTT). Women with two values meeting or exceeding O'Sullivan and Mahan's values on the follow-up OGTT should be considered to have gestational diabetes. The American Diabetes Association endorsed the conference recommendations. The American College of Obstetricians and Gynecologists recommends the same screening and diagnostic values; however, it recommends selected screening only for women under age 30.
Keep in mind that O'Sullivan and Mahan chose their cutoffs for convenience in follow-up. No threshold has ever been demonstrated for onset or marked increase in fetal complications below levels diagnostic of diabetes. Instead of raising questions about the validity of GD testing, this lack of correlation with complications has led some researchers to lobby for a lowering of diagnostic thresholds, which would label even more women gestational diabetics.
-Henci Goer, Obstetic Myths Versus research Realities, A Guide to the Medical Literature, Bergin & Garvey 1995
Reprinted from Midwifery Today E-News (Vol 1 Issue 47, Nov 19, 1999)
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