Many female oral contraceptives contain oestrogen and progesterone. How would these hormones help to prevent conception from occurring?

Progesterone prevents follicular development and prevent ovulation. Progesterone exerts a negative feedback effect on the hypothalamus to decrease GnRH release and therefore decrease the secretion of FSH and LH. Without FSH no follicle develops, and without the LH surge ovulation could not occur. Oestrogen also has a negative feedback effect that decreases FSH secretion. Another effect of progesterone is to inhibit sperm from penetrating through the cervix by changfing the cervical mucous.