Diabetes mellitus is a group of metabolic disorders characterized by hyperglycemia due to defects in insulin production, insulin action, or both. Type 1 is caused by autoimmune destruction of pancreatic β-cells (no insulin produced), while Type 2 — the most common form — results from insulin resistance combined with inadequate insulin secretion. Gestational diabetes occurs during pregnancy, resembles Type 2, usually resolves after delivery, but significantly raises the mother's risk of developing Type 2 diabetes later in life.