Pigs belong to poliestral animals because their reproductive cycle is repeated several times per year. The characteristics of the menstruation involves repeated cycles of the integrated cellular reproductive axis. The morphological changes of the reproductive organs such as uterus, fallopian tubes and vagina accompany the progress of the menstrual cycle. The understanding of the estral cycle mechanism and the monitoring of the estrus phase in female individuals affect the efficiency of pig farming because they can help to resolve the problems in sow reproduction that cause financial loss in pig production.