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Is gestational diabetes a health risk?
The theory that GD could have the same adverse effects of diabetes was faulty on its face, because GD does not share the risk factors of either type of true diabetes. In Type I diabetes, extremes of low and high blood sugar early in pregnancy can cause malformations or miscarriage. GD women make normal or above-normal amounts of insulin and have normal blood-sugar metabolism in the first trimester (22). Either Type I or II, long-standing diabetes can damage maternal blood vessels and kidneys, causing hypertension or kidney complications. These can in turn jeopardize the fetus. Gestational diabetics do not have long-standing diabetes. The one problem GD shares with both types is that chronic hyperglycemia can overfeed the fetus, resulting in a big baby. This is generally defined as a birth weight of more than 8 lbs. 13 oz. (4,000 grams) or a birth weight in the upper ten percent for length of pregnancy (large for gestational age).
Theory aside, the studies designed to test it had significant weaknesses. They included women who were known diabetics prior to pregnancy. They selected women for glucose testing based on such risk factors as prior stillbirth, current hypertension, or extreme overweight, indications that alone could explain poorer outcomes (12). They failed to account for compounding factors, such as that glucose intolerance associates with increasing maternal weight and age, which themselves strongly predict large babies and maternal hypertension. Finally, they used management protocols that increased risks such as starvation diets, early induction and withholding nourishment from the newborn (18). Despite these flaws, researchers concluded that mildly deviant glucose values in pregnancy caused serious harm.
We now know that GD doesn’t increase the risk of stillbirth or congenital malformations (4). A couple of modern studies have concluded otherwise, but they didn’t take into account that women with high blood sugar are more likely to have other risk factors for poor outcome, or that some women had undiagnosed diabetes prior to pregnancy (17,24). Indeed, the fact that these studies were of women whose blood sugar had been normalized by treatment proves that GD is not the culprit. Besides, GD testing and treatment could not affect the incidence of congenital malformations under any circumstances, because testing isn’t done until the third trimester. By that time, the baby is long since fully formed.
We also know that maternal glucose level correlates poorly with birth weight. While GD somewhat increases the odds of having a baby weighing in the upper ten percent (16,36), most of this results from GD’s association with other factors, in particular, maternal weight (10,13,21,28,43,57).
Other supposed risks of GD are preeclampsia, glucose intolerance in the child and childhood obesity. As before, GD is only found in company with these complications; it doesn’t cause them. For example, studies show that blood glucose level plays little if any role in high-weight children compared with maternal weight before pregnancy (8,25). Also, as before, normalizing blood sugar fails to prevent these problems, which absolves GD (42,44-45,53).
All this being said, there is a needle in the haystack. About one in a thousand pregnant women tested will have sugar values in the range of true diabetes (2). These women may have been diabetic before pregnancy and not known it, or pregnancy may have been enough of a metabolic stress to tip them into diabetes. These women may benefit from being identified and treated.