How does birth control work (the pill form)?

Most birth control pills are "combination pills" containing a combination of the hormones estrogen and progesterone to prevent ovulation (the release of an egg during the monthly cycle). A woman cannot get pregnant if she doesn't ovulate because there is no egg to be fertilized. The Pill also works by thickening the mucus around the cervix, which makes it difficult for sperm to enter the uterus and reach any eggs that may have been released. The hormones in the Pill can also sometimes affect the lining of the uterus, making it difficult for an egg to attach to the wall of the uterus. Most combination pills come in either a 21-day pack or a 28-day pack. One hormone pill is taken each day at about the same time for 21 days. Depending on your pack, you will either stop taking birth control pills for 7 days (as in the 21-day pack) or you will take a pill that contains no hormones for 7 days (the 28-day pack). A woman has her period when she stops taking the pills that contain hormones.  more