月經是生理女性沒有懷孕時子宮內膜從陰道排出體外的自然機制。在月經週期內，子宮內膜會增厚，成為適合囊胚著床（胚胎植入子宮內膜）的環境。若卵子沒有受精，子宮內膜便會脫落，從陰道排出，形成月經。大約一半的經血為子宮內膜脫落時血管撕裂產生的血液，另外一半為子宮內膜的組織、子宮頸黏液及陰道分泌物等。

月經週期為21至35日不等，一般為28日；而月經來潮一般持續3至5日（2至7日皆為正常範圍）。整個月經週期除了排卵（以月經來潮第1日即月經週期第1日計算，排卵大概在週期第12至16日）和月經來潮外，亦會伴隨荷爾蒙變化波動，引致經期前情緒不穩，甚至頭痛、失眠、水腫等生理反應。經期前的生理症狀因人而異，統稱經前綜合症(PMS)。

Menstruation is a natural process of the uterus shedding its lining when no pregnancy occurs. During the menstrual cycle, the endometrium (inner lining of the uterus) thickens to become an optimal environment for the implantation and growth of an embryo. If no pregnancy occurs, the endometrium would be shed and discharged through the vagina. About half of the menstrual blood is the blood produced by the tearing of the blood vessels when the endometrium is shed, and the other half is the tissue of the endometrium, cervical mucus and vaginal secretions.

The menstrual cycle is 21 to 35-day long (on average 28-day), while bleeding usually lasts for 3-5 days (2-7 days is still considered normal). Apart from ovulation which happens on days 12 to 16 in a 28-day cycle (counting the first day of menstruation as day 1) and menstruation, the menstrual cycle also brings us various hormonal changes. These hormonal changes can be accounted for mood swings, headache, insomnia and edema etc., which are also referred to as PMS (premenstrual syndrome).