After ovulation, the remaining parts of the Graafian follicle transform into the corpus luteum. The corpus luteum secretes large amounts of progesterone, which is essential for the maintenance of the endometrium, necessary for implantation and pregnancy . While the ovary secretes relaxin in later stages of pregnancy , and the placenta secretes hCG , the main function of the corpus luteum defined in the text is the production of progesterone.