During ovulation, the follicle breaks open and colapses under the continuous influence of luteinizing hormone (LH). It begins to enlarge and forms a yellowish structure, called corpus lutenum or yellow body. The corpus luteum plays an important role in the preparation of the endometrium for the implantation of fertilized egg by secreting estrogen and progesterone hormones. The progesterone speeds up the thickening of the lining of the uterus and thus increases its blood supply.