According to the sources, menstruation occurs only if the released ovum is not fertilised . In the absence of fertilisation, the corpus luteum degenerates, causing the disintegration of the endometrium and the start of a new menstrual cycle . If fertilisation occurs, it triggers the start of pregnancy, during which all events of the menstrual cycle stop and no menstruation takes place . While the retention of the corpus luteum and the maintenance of the endometrial lining are the immediate physiological states that prevent bleeding, they are both downstream consequences of the root cause, which is the fertilisation of the ovum .