Evidence is insufficient to permit judgement of which is the best way to identify women who have gestational diabetes.
Insulin is a hormone produced in the pancreas that enables cells to absorb glucose to turn it into energy. During pregnancy, maternal resistance to insulin develops so that glucose can more easily cross the placenta to the growing foetus. Resistance to insulin becomes apparent in the second trimester and declines progressively to term. Insulin resistance returns to normal after pregnancy, usually within six weeks of birth. For about seven in every 100 pregnant women, resistance to insulin is excessive, and the woman’s blood sugar becomes too high. This is known as gestational diabetes.
If gestational diabetes develops and resistance to maternal insulin becomes too great, high fetal blood insulin can develop and may cause increased growth and fat deposits, increased birthweight and perinatal complications. The woman and her baby can be harmed by high blood sugar levels if untreated, and harmful effects may remain after pregnancy. Growing knowledge shows that infants of women who have had gestational diabetes are at increased risk of obesity and high blood pressure later in life.
Lowering blood sugar levels can reduce the harmful effects, but women can receive treatment only if they are clearly identified. Several tests are used to find out whether a woman has gestational diabetes. Most involve giving a sugary drink and taking a series of blood tests over one to three hours; this is known as the oral glucose tolerance test (OGTT). Limitations of the OGTT are that it requires women to fast from the night before, to drink a glucose solution and to wait for two or three hours before having the final blood test. Other tests have been proposed that do not require this level of involvement by the pregnant woman or by healthcare staff. Using different tests and different criteria (glucose thresholds that define gestational diabetes) will influence identification of women diagnosed. This review aimed to determine the best way of identifying women with gestational diabetes. We identified six small randomised trials (involving 694 women) of unclear quality, which compared different ways of giving a glucose load. None evaluated the important question of what time during pregnancy is best to test women for gestational diabetes, and no studies compared the 75-gram or 100-gram OGTT with other strategies. Large well-designed trials are needed to provide information about the best way to identify women with gestational diabetes.