Step 1: (a) Fertilised - The corpus luteum is maintained by the hormone hCG. It continues to secrete Progesterone, which maintains the endometrium for pregnancy.
Step 2: (b) Not fertilised - In the absence of fertilization, the corpus luteum degenerates.
Step 3: Result - Its degeneration into the Corpus albicans leads to a drop in progesterone levels, causing the breakdown of the endometrium and the onset of menstruation.