Gestational diabetes mellitus (GDM) is the development if diabetes type II during pregnancy in women who were not previously affected by diabetes. The prevalence is estimated to be between approximately 4% to 11% of all pregnancies (although studies vary on this). However, as with regular (non-gestational) diabetes, prevalence over the last few decades has increased by as much as 30% and is likely still rising due to more sedentary and less active lifestyles. The causes of of GDM are not well understood, but it is believed that increased BMI of the mother prior to conception, genetic factors, and rise in placental hormone levels causing insulin resistance play an important role. If monitored and treated properly it is usually not life threatening for either mother or fetus, and resolves after pregnancy. Symptoms and  treatments are essentially similar to normal (non-gestational) diabetes. If you are pregnant and don't have GDM, it is important to consider preventative measures, which usually don't significantly impact quality of life but can considerably reduce the risk of developing GDM later in the pregnancy.