Use of progesterone in preventing ovulation?
When progesterone alone is used as a contraceptive it is assumed that the oestrogen level is endogenously maintained by the follicular cells. 


Yes mainly oestrogen and only little does progesterone inhibits LH or FSH secretion.

LH surge is the principle stimulator of ovulation.


  However,experimental work has suggested that immediately before the LH surge occurs, there is probably a sudden depression of estrogen...and this might be ...that leads to LH surge
  [source: Textbook of medical physiology A South Asian Edition- Guyton &amp; Hall]


If progesterone or progesterone-oestrogen combinations be given before ovulation then feed back inhibition of LH surge occurs.This would lead to contraception by anovulation.




Progesterone is only known to proliferate the endometrial lining.


However, administration of progesterone or oestrogen -progesterone combinations after ovulatory phase helps to achieve high concentration of these hormones ,much before the normal time required to attain peak levels due to endogenous secretions alone.

This helps to depress the FSH -LH secretion by feedback inhibition very quickly causing involution of the corpus luteum and early shedding of the endometrium termed as breakthrough bleeding

REFERENCE:


reference site
Textbook of Medical Physiology A South Asian Edition-Guyton &amp; Hall