The oral contraceptive pill, informally called "the pill," contains an estrogen and progesterone, or a synthetic mimic of this hormone. The combination of these two hormones, taken in the pill and circulating in the blood, works because

the hormones create an acid condition in the vagina that acts as a spermicide

the hormones prevent the proliferative or secretory phase of the menstrual cycle and the embryo will have no place to implant

the hormones cause the woman's body to mimic the activities of a pregnancy and disrupt the ovarian cycle, preventing ovulation

the hormones cause a thickening of the mucus plug at the cervix that prevents sperm from penetrating the womb and reaching the ovulated egg