The normal menstrual cycle can be divided into two phases: follicular and luteal, which are
separated by ovulation and bookended by the first day of menstrual bleeding. The follicular
phase is dominated by the development of the preovulatory follicle, resulting in estrogen-
stimulated endometrial proliferation, whereas the corpus luteum (CL) of its namesake luteal
phase produces progesterone, which inhibits endometrial proliferation and determines
endometrial receptivity. Without both phases working in series, natural reproduction is not …