Gestational diabetes mellitus (GDM) is a form of glucose intolerance first detected during pregnancy. It is diagnosed through screening for clinical risk factors and abnormal glucose tolerance, typically mild and asymptomatic. GDM is linked to the same physiological and genetic abnormalities as diabetes outside pregnancy, making it a valuable opportunity to study diabetes pathogenesis and prevention. Historically, GDM was considered a transient condition, but research has shown that glucose intolerance during pregnancy is associated with increased risk of developing diabetes postpartum. The American Diabetes Association (ADA) currently recommends diagnostic criteria based on O'Sullivan's values, which are used to identify pregnancies at risk for perinatal morbidity and long-term offspring obesity and glucose intolerance. GDM is generally asymptomatic and is diagnosed through two-step screening, followed by a 2- or 3-hour oral glucose tolerance test (OGTT) for at-risk women. The frequency of GDM varies by ethnicity and diagnostic criteria. Most women with GDM have chronic insulin resistance, which contributes to the risk of type 2 diabetes. However, a small minority may have diabetes-like glucose levels outside pregnancy. The etiology of GDM involves insulin resistance and β-cell dysfunction, with insulin resistance being a key factor. GDM can be associated with autoimmune diabetes, monogenic diabetes, or insulin resistance. Autoimmune diabetes is characterized by the presence of anti-islet cell and anti-GAD antibodies, while monogenic diabetes is linked to mutations in genes such as glucokinase, hepatocyte nuclear factor 1α, and insulin promoter factor 1. Insulin resistance in GDM is often exacerbated by pregnancy, leading to increased insulin resistance and impaired glucose metabolism. Insulin resistance is associated with increased risk of type 2 diabetes, and studies have shown that treatment of insulin resistance can reduce the risk of diabetes. GDM is also linked to long-term risk of type 2 diabetes, with studies showing that approximately 70% of women with GDM develop diabetes within 10 years. Prevention strategies include lifestyle modification and metformin, which have been shown to reduce the risk of type 2 diabetes in high-risk individuals. Clinical management of GDM focuses on normalizing maternal glucose levels and minimizing fetal overnutrition. Postpartum care should focus on reducing diabetes risk and detecting and treating diabetes that develops. Genetic testing for monogenic diabetes is primarily a research tool, but clinical tests are being developed. Future research aims to better understand the mechanisms of GDM and its progression to diabetes, with the goal of improving prevention and treatment strategies.Gestational diabetes mellitus (GDM) is a form of glucose intolerance first detected during pregnancy. It is diagnosed through screening for clinical risk factors and abnormal glucose tolerance, typically mild and asymptomatic. GDM is linked to the same physiological and genetic abnormalities as diabetes outside pregnancy, making it a valuable opportunity to study diabetes pathogenesis and prevention. Historically, GDM was considered a transient condition, but research has shown that glucose intolerance during pregnancy is associated with increased risk of developing diabetes postpartum. The American Diabetes Association (ADA) currently recommends diagnostic criteria based on O'Sullivan's values, which are used to identify pregnancies at risk for perinatal morbidity and long-term offspring obesity and glucose intolerance. GDM is generally asymptomatic and is diagnosed through two-step screening, followed by a 2- or 3-hour oral glucose tolerance test (OGTT) for at-risk women. The frequency of GDM varies by ethnicity and diagnostic criteria. Most women with GDM have chronic insulin resistance, which contributes to the risk of type 2 diabetes. However, a small minority may have diabetes-like glucose levels outside pregnancy. The etiology of GDM involves insulin resistance and β-cell dysfunction, with insulin resistance being a key factor. GDM can be associated with autoimmune diabetes, monogenic diabetes, or insulin resistance. Autoimmune diabetes is characterized by the presence of anti-islet cell and anti-GAD antibodies, while monogenic diabetes is linked to mutations in genes such as glucokinase, hepatocyte nuclear factor 1α, and insulin promoter factor 1. Insulin resistance in GDM is often exacerbated by pregnancy, leading to increased insulin resistance and impaired glucose metabolism. Insulin resistance is associated with increased risk of type 2 diabetes, and studies have shown that treatment of insulin resistance can reduce the risk of diabetes. GDM is also linked to long-term risk of type 2 diabetes, with studies showing that approximately 70% of women with GDM develop diabetes within 10 years. Prevention strategies include lifestyle modification and metformin, which have been shown to reduce the risk of type 2 diabetes in high-risk individuals. Clinical management of GDM focuses on normalizing maternal glucose levels and minimizing fetal overnutrition. Postpartum care should focus on reducing diabetes risk and detecting and treating diabetes that develops. Genetic testing for monogenic diabetes is primarily a research tool, but clinical tests are being developed. Future research aims to better understand the mechanisms of GDM and its progression to diabetes, with the goal of improving prevention and treatment strategies.