Menstruation begins when
- A. oestrogen rises sharply
- B. follicle stimulating hormone falls to zero
- C. the corpus luteum degenerates and progesterone falls
- D. the blastocyst implants in the uterine wall
Explanation
Without implantation the corpus luteum breaks down, progesterone withdrawal removes the support of the endometrium, its blood vessels constrict and the lining is shed as the menstrual flow. The falling progesterone also releases the pituitary from inhibition, so FSH rises again and the next cycle begins. Implantation has the opposite effect, since it keeps the corpus luteum alive and prevents menstruation.