Birth control pills prevent pregnancy through several methods, mainly by stopping ovulation. If no egg is released , there is nothing to be fertilized by sperm, and woman cannot get pregnant. 

Most birth control pills contain two female hormones, the estrogen and progestin. these are called combination pills. the hormones in the pill work by keeping eggs from leaving the ovaries. Pregnancy can't happen if there's no egg to join with sperm.

Estrogen in the pill will stop the pituitary gland from producing follicle stimulating hormone (FSH) and luteinizing hormone (LH) in order to prevent ovulation.

Progestin will stop the pituitary gland from producing LH in order to prevent egg release and thicken the cervical mucus to hinder sperm movement.