Corpus luteum produces large amounts of

After ovulation, when the corpus luteum forms, it secretes large amounts of progesterone. This progesterone helps in thickening the lining of the uterus to prepare for the implantation of the fertilized egg. If fertilization occurs, the corpus luteum persists and secretes progesterone. If no fertilization occurs, the corpus luteum starts to degenerate, and gets converted into corpus albicans, which cannot secrete progesterone. As a result, the progesterone level drops down. Due to the lack of progesterone, the overgrown endometrium now begins to break, causing bleeding.