How do birth control pills avoid pregnancy?
Answers: A spike of a certain hormone call LH causes you to ovulate. Giving the body specific doses of estrogen and/or progesterone found surrounded by BC pills prevents the spike of LH, and you do not ovulate your egg.
I guess you are talking roughly speaking the oral contraceptive pill. There are different types of pills, containing different substances. The most commonly used are the combined oestrogen and progestagen pill. Women get different level of hormones during the menstrual cycle which cause the ovum to develop within the ovary, to be then released during ovulation. The pill act in a channel that it inhibits this process and doesn't let ovulation crop up. Therefore if there is ovum, pregnancy can not materialize.