Question 4
Why does the uterine wall break down at the start of the menstrual phase?
The right answer is the one marked above.
No fertilisation occurred, so the level of progesterone falls. The wall that had developed breaks down and passes out along the vagina with blood — the menstrual flow, lasting about four days.
Fertilisation has the opposite effect: the wall is kept and the embryo develops in it. Rising oestrogen is what rebuilds the wall afterwards, in the proliferative phase, and the follicle bursts a fortnight earlier.