Researchers don't yet know exactly why some women develop gestational diabetes. To understand how gestational diabetes occurs, it can help to understand how pregnancy affects your body's normal processing of glucose.
Your body digests the food you eat to produce sugar (glucose) that enters your bloodstream. In response, your pancreas — a large gland behind your stomach — produces insulin. Insulin is a hormone that helps glucose move from your bloodstream into your body's cells, where it's used as energy.
During pregnancy, the placenta that connects your growing baby to your blood supply produces high levels of various other hormones. Almost all of them impair the action of insulin in your cells, raising your blood sugar. Modest elevation of blood sugar after meals is normal during pregnancy.
As your baby grows, the placenta produces more and more insulin-blocking hormones. In gestational diabetes, the placental hormones provoke a rise in blood sugar to a level that can affect the growth and welfare of your baby. Gestational diabetes usually develops during the last half of pregnancy — sometimes as early as the 20th week, but usually not until later.