Gestational diabetes is a temporary form of diabetes which begins during pregnancy and usually disappears after delivery. It affects about four percent of all pregnant women - that equates to 135,000 cases of gestational diabetes in the United States each year.

As with diabetes, gestational diabetes means that the body is not able to make and use enough insulin. In gestational diabetes the insulin is blocked from doing its job – this is called insulin resistance. Without enough insulin, glucose cannot move from the blood to the body’s cells where it is converted to energy. Instead, it remains in the blood where it builds up to high levels. This is called hyperglycemia.

Insulin resistance in women with gestational diabetes is actually caused by hormones in the placenta interfering with insulin. The function of the placenta in pregnancy is to supply the growing baby with nutrients and water from the mother's circulation. In this process, the placenta produces a number of hormones which have a critical role in maintaining the pregnancy. Unfortunately, some hormones produced in the placenta, such as estrogen, cortisol, and human placental lactogen (HPL), have the effect of blocking insulin. This usually begins in weeks 24-28 of pregnancy and worsens as the pregnancy develops and the placenta becomes larger.

Any woman can develop gestational diabetes during pregnancy, though there are a number of high risk factors. These include:

• Obesity
• A family history of diabetes
• Previous stillbirth
• Birth defects in previous children
• Having had a large baby previously
• African or Hispanic ancestry
• Recurrent infections
• Too much amniotic fluid
• Being older than 25 (small indicator)

The most common screening test for gestational diabetes is an oral glucose challenge test (OGCT) taken 26-28 weeks into the pregnancy, although those with high risk factors should test earlier. In the OGCT test the patient is given 50g of a glucose solution to drink, and then a blood sample is taken to measure the level of glucose in the blood. This is a simple test, as there is no special preparation necessary, and no need to fast before the test. If the patient’s blood sugar level is higher than 140 mg/dL (milligrams of glucose per deciliter of blood) then the test is considered positive, and further testing is done to confirm a diagnosis.

Further testing usually involves a two hour oral glucose tolerance test (OGTT). For this test, overnight fasting is necessary. After fasting, a blood sample is taken to measure the fasting blood sugar level and then the patient drinks 75g of a glucose solution. (This glucose drink can make some patients feel nauseous, as it is very sweet). A second blood test is taken three hours later. If results are over 140 mg/dL then the patient will be treated for gestational diabetes.

Current research: Research published in the February 2004 issue of Diabetes Care suggests that measuring the size of the unborn baby may provide a more accurate indication of gestational diabetes than testing the mother’s blood glucose levels. Using ultrasound technology, researchers found that giving insulin only to mothers whose baby’s abdominal size was in the 75th percentile or higher produced better results than giving it to all mothers whose blood glucose levels were higher than normal.