There is not enough evidence to judge 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 in order to turn it into energy. During pregnancy maternal resistance to the action of insulin develops so that glucose can be more easily transported across the placenta to the growing fetus. 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 the 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 the resistance to maternal insulin becomes too pronounced, fetal hyperinsulinaemia can cause accelerated growth with fetal adiposity, increased birthweight and perinatal complications. The woman and her baby can be harmed by the high blood sugar levels if untreated, and there may be adverse effects after pregnancy. Evidence is increasing that the offspring are at increased risk of obesity and high blood pressure in later life.
Lowering blood sugar levels can reduce the harmful effects, but women will only receive treatment if they are correctly identified early enough in pregnancy. Several tests are used to find out if a woman has gestational diabetes. Most involve giving the woman a very sugary drink or food, and taking a series of blood sugar 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, drink a glucose solution and wait for two or three hours before having the final blood test. Therefore, other tests have also been proposed that do not require this level of involvement by the pregnant women or healthcare staff. These include taking just one blood test after an overnight fast or taking just one test with no fasting. This review aimed to determine what was the best way of identifying women with gestational diabetes. We identified five small randomised trials (involving 578 women) of unclear quality, comparing different ways of giving a glucose load. None evaluated the important question of when the best time is during pregnancy to test women for gestational diabetes or compared the 75 g or 100 g oral glucose tolerance test with other strategies. Large well-designed trials are needed to provide information about the best way of identifying women who have gestational diabetes.