Gestational diabetes mellitus (GDM) is a carbohydrate intolerance of varying degrees of severity which starts or is first recognized during pregnancy. The definition applies regardless of whether insulin is used for treatment or if the condition persists after pregnancy. It does not exclude the possibility that unrecognized glucose intolerance may have antedated the pregnancy.

Increased maternal glucose (blood sugar) levels are associated with an increased rate of complications in the baby, including large size at birth, birth trauma, hypoglycaemia (low blood sugar), and jaundice. Maintaining control of blood sugar levels significantly reduces the risk to the baby.

Women who have had GDM have an increased risk of developing type 2 diabetes in later years. GDM is also associated with increased risk of obesity and abnormal glucose metabolism during childhood and adult life in the offspring.