Definition of Corpus luteum:
During ovulation the Graffian follicle ruptures and releases the secondary oocyte. The remaining cells of the Graffian follicle form the corpus luteum.
In case of fertilization:
When fertilization takes place corpus luteum acts as a temporary endocrine gland and produces progesterone, which helps to prepare the endometrium for implantation. So corpus luteum is maintained throughout the pregnancy.
In case of absence of fertilization:
In case of absence of fertilization corpus luteum degenerates to white body, as the maintenance of endometrium is not required. This causes the disintegration of endometrium leading to menstruation.