Gestational diabetes occurs only during pregnancy, and affects about 4 percent of pregnant women, says the American Diabetes Association.

Though scientists aren't certain of its exact cause, it's thought that hormones from the placenta make it difficult for the mother's body to process insulin.

Left untreated, the baby can get too much blood sugar from the mother, possibly leading to a high birth weight, difficult delivery and an above-normal risk of developing type 2 diabetes later in life, the association says.

Women with gestational diabetes typically are put on a special diet and a regular exercise regimen to help keep blood glucose levels under control. The ADA says some women may also need to take insulin shots and test their blood glucose levels regularly during pregnancy.