The internal female reproductive organs include the uterus, ovaries, cervix, and vagina. These organs are necessary to produce a successful pregnancy. To prevent pregnancy, birth control pills affect how these organs function.

Follicle stimulating hormone (FSH) and luteinizing hormone (LH) travel through the bloodstream and stimulate the ovary into producing a ripe egg ready for fertilization by sperm during a normal ovulation cycle.

During a normal menstrual cycle, hormones stimulate the ovary, causing an egg to ripen. The uterine lining thickens, preparing itself for implantation of a fertilized egg, and the cervical mucus thins to help sperm reach the egg.

Changing levels of estrogen in the body causes the pituitary gland to release LH and FSH, stimulating the ovary to produce a ripe egg.

The steady low levels of estrogen in birth control pills suppress FSH and LH, "fooling" the pituitary gland into thinking a woman is pregnant. Ovulation will then not occur, therefore preventing pregnancy.

The progesterone in birth control pills also creates a thick cervical mucus, making it difficult for sperm to reach the uterus. It also impedes an egg from attaching itself to the uterine lining (endometrium) because of changes in the cellular structure of the lining.