Birth control pills contain progestin and/or ethinyl estadiol, synthetic hormones that mimic the effects of the naturally produced female hormones progesterone and estrogen.
During the menstrual cycle, a drop in estrogen levels trigger the pituitary gland to produce Luteinizing hormone, which then triggers ovulation. At the same time, a drop in progesterone levels triggers changes in the lining of the cervix and uterus to allow fertilization and implantation of a fertilized egg.
Contraceptives prevent these effects by keeping hormonal levels more constant. Progestin-only pills cause the mucus lining of the cervix to thicken so that sperm will be unable to reach the fertilized egg. They also prevent the uterine lining from softening so that a fertilized egg is unable to implant. Pills with a combination of progestin and ethyl estradiol also inhibit the release of luteinizing hormone and prevent ovulation. When used correctly, the combination of the two hormones is 99% effective in preventing pregnancy.
The birth controll pills contain synthetic derivative of estrogen and progesteron, hence, taking birth controll pills the levels of progesterone are maintained increased.
These increased levels of progesterone keep secretions of FSH and LH of happening. If the secretion of FSH and LH is stopped, hence, the egg follicles that secrete estrogen hormones are not formed and the ovulation does not happen.
The birth controll pills imitate the state of pregnancy, when the levels of progesterone and estrogen remain increased to protect the fertilized egg.