Progesterone
Progesterone is required for maintenance of the endometrial lining of the uterus where the blastocyst is implanted.

If the released egg is not fertilized, the corpus luteum regress, and the production of progesterone drops. The uterine lining is sloughed off, completing the monthly uterine cycle. Following the menstrual period, the ovarian and uterine cycles begin anew, with the development of an ovarian follicle, and the preparation of the uterus for blastocyst implantation.

Oral contraceptives
Oral contraceptives normally contain progesterone, or progesterone in combination with other estrogens. By maintaining a slightly elevated level of progesterone via oral contraceptives (or injectible contraceptives), the signals from the brain to the ovaries to initiate the ovarian cycle that lead to follicle maturation are repressed, and ovulation does not occur.