月经周期，又称作经期、生理期，是人类女性在生理上的循环周期，发生在其他的哺乳类动物则是经历动情周期。有性繁殖可能与否和月经周期有着密切的关系。

对人类来说，女性的月经周期从21到35天不等，一般在28天左右。依据发生地点是卵巢还是子宫，每次月经都可以分为数个阶段。卵巢阶段可以分为卵泡期、排卵和黄体期，而子宫阶段则可以分为月经、增生期和分泌期。每个阶段都由内分泌系统控制，因而也可以使用激素避孕的方法来防止发生。

人类与其它物种不同，相对于其他雌性动物在排卵期有明显的外表特征，排卵期的女性外表变化相当不明显，一个女人或许会感觉到她的排卵，但是对于其他人来说则是难以识别的，这称作隐性排卵；这个特点则是具有社会生物学意义的。相反，其它物种通常通过热度来感受信息。类人猿是除人类之外唯一一种拥有隐性排卵的哺乳动物。

The menstrual cycle is the regular natural change that occurs in the female reproductive system like the uterus and ovaries that make pregnancy possible. The cycle is required for the production of ovocytes, and for the preparation of the uterus for pregnancy. Up to 80% of women report having some symptoms during the one to two weeks prior to menstruation. Common symptoms include acne, tender breasts, bloating, feeling tired, irritability and mood changes. These symptoms interfere with normal life and therefore qualify as premenstrual syndrome in 20 to 30% of women. In 3 to 8%, they are severe.