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What is gestational diabetes?
Gestational diabetes (GD) simply means elevated blood sugar during pregnancy. To understand it, you must first understand the normal changes in pregnancy metabolism (34). When you are pregnant, certain hormones make your insulin less effective at transporting glucose, the body’s fuel, out of your bloodstream into your cells. This increases the amount of circulating glucose, making it available to your baby for growth and development. This “insulin resistance” increases as pregnancy advances. As a result, your blood glucose levels after eating rise linearly throughout pregnancy. By the third trimester, you will tend to have higher blood glucose levels after eating than nonpregnant women (hyperglycemia), despite secreting normal and above normal amounts of insulin. During overnight sleep, the excess insulin has a chance to mop up, which causes morning glucose levels to be lower on average than in nonpregnant women (hypoglycemia).
In the 1950s, some researchers wondered whether sugar values at the high end of the range for pregnancy would predict the development of diabetes later in life. They tracked a population of women and in 1964, they reported that, yes, it did (40). The extra stress of pregnancy revealed a woman’s “prediabetic” status. This shouldn’t have surprised anyone, because high-weight women are much more likely to have higher glucose values in pregnancy than average-weight women and to eventually develop diabetes. However, doctors knew diabetes posed grave threats to the unborn baby, so they worried that glucose levels that were high, but not in the diabetic range, might also do harm. This concern launched what eventually became an avalanche of studies that ended by defining a whole new category of pregnancy complication called “gestational diabetes,” although “glucose intolerance of pregnancy” would be a more accurate description. Those studies, and their premise, were fundamentally flawed.