Gestational diabetes develops in women during pregnancy because the mother’s body is not able to produce enough insulin. Insulin is a hormone that enables the body to break down sugar (glucose) to be used as energy. Without sufficient insulin the amount of sugar in the blood rises.

High blood sugar levels in the mother’s body are passed through the placenta to the developing baby. This can cause health problems.

Gestational diabetes usually begins in the second half of pregnancy, and goes away after the baby is born. This makes it different to the more common forms of diabetes which, once they occur, are permanent.

Gestational diabetes mellitus appears in about 2%–5% of all pregnancies. It is temporary and fully treatable, but, if untreated, it may cause problems with the pregnancy, including macrosomia (high birth weight) of the child. It requires careful medical supervision during the pregnancy. In addition, about 20%–50% of these women go on to develop type 2 diabetes.

The cause of gestational diabetes is unknown. It is thought that the hormones produced during pregnancy may block the action of insulin. Gestational diabetes can happen if the mother’s body can’t produce enough extra insulin to counteract this blocking effect.