The Birth Control Pill

The Birth Control Pill

What Is It?
The birth control pill (also called "the Pill") is a daily pill that contains hormones to change the way the body works and prevent pregnancy. Hormones are chemical substances that control the functioning of the body's organs. In this case, the hormones in the Pill control the ovaries and the uterus.
How Does It Work?
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. Some women prefer the 28-day pack because it helps them stay in the habit of taking a pill every day.
There is also a type of combination pill that decreases the frequency of a woman's period by supplying a hormone pill for 12 weeks and then inactive pills for 7 days. This decreases the number of periods to one every 3 months instead of one every month.
Another kind of pill that may change the number of monthly periods is the low-dose progesterone pill, sometimes called the mini-pill. This type of birth control pill differs from the other pills in that it only contains one type of hormone — progesterone — rather than a combination of estrogen and progesterone. It works by changing the cervical mucus and the lining of the uterus, and sometimes by affecting ovulation as well. The mini-pill may be slightly less effective at preventing pregnancy than combination pills.
The mini-pill is taken every day without a break. A girl who is taking the mini-pill may have no period at all or she may have irregular periods. For the minipill to work, it must be taken at the same time every day, without missing any doses.
Any type of birth control pill works best when it is taken every single day at the same time of day, regardless of whether a girl is going to have sex. This is especially important with progesterone-only pills.
For the first 7 days of taking the Pill, a girl should use an additional form of contraception, such as condoms, to prevent pregnancy. After 7 days, the Pill should work alone to prevent pregnancy. But continuing to use condoms will protect againstsexually transmitted diseases (STDs).
If pills are skipped or forgotten, a girl is not protected against pregnancy and she will need a backup form of birth control, such as condoms. Or she will need to stop having sex for a while. Do not take a friend's or relative's pills.

http://kidshealth.org/teen/sexual_health/contraception/contraception_birth.html