Why there is no menstruation after fertillization

After fertilization, the corpus luteum (formed from the remnant graafian follicle after ovulation) is retained to secrete progesterone which is essential for maintainence of the endometrium to faciliate further embryonal development and thus menstruation ceases. But in the absence of fertilization, the corpus luteum degenerates which causes disintegration of the endometrium leading to menstruation.