月经（英语：menstruation, period, monthly，俗称好朋友、大姨妈等），系指血液或黏膜定期从子宫内膜经阴道排出体外的现象。高达80%女性曾表示在月经开始前出现部分症状。常见症状包含长出粉刺、乳房变得柔软、感到疲惫、容易发怒与情绪改变。由于这些症状会干扰正常生活，因此被称为经前综合症；20-30%女性会得到此症，其中的3-8%则较为严重。

月经通常在12-15岁之间首次出现，该次月经被称做初潮；然而，初潮可能会提早于8岁出现。已开发国家的初潮平均年龄比开发中国家还早。从一段经期开始到下一段经期开始的间隔（月经周期），在青少年女性时期为21-45天，在成年女性则为21-31天，平均约为28天。月经在更年期后结束，通常发生于45-55岁时。月经来临时，经血会持续流出约2-7天。

Menstruation, also known as a period or monthly, is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. Up to 80% of women report having some symptoms prior to menstruation. Common signs and symptoms include acne, tender breasts, bloating, feeling tired, irritability, and mood changes. These may interfere with normal life, therefore qualifying as premenstrual syndrome, in 20 to 30% of women. In 3 to 8%, symptoms are severe.