The menstrual cycle
Over the monthly cycle the womb lining gradually thickens. After ovulation, when fertilization does not occur and so no egg is implanted, the womb lining is expelled at the end of the menstrual cycle.
This process manifests itself in a monthly period, although how heavy the bleeding is, how long it lasts and how painful it is varies.
The diagrams show how the womb lining thickens and is expelled during the menstrual cycle.