Gestational diabetes develops near the end of the second trimester of pregnancy. This happens when the pancreas can't make enough insulin to meet the extra demands of pregnancy. There are about 135,000 cases of gestational diabetes in the United States each year - about 4% of all pregnant women. It is diagnosed with a blood test. In most cases, diet and exercise are enough to treat gestational diabetes. Once the pregnancy ends gestational diabetes goes away in 90% of cases.