Center for Young Women's Health

Birth Control Pills


  • There are many different kinds of birth control pills (BCP's).
  • BCP's contain estrogen and progesterone.
  • BCP's are also called "oral contraceptive pills" or "hormone pills".


current guideBirth Control Pills How to take BCP's
Side Effects Frequently Asked Questions
Medical Benefits Birth Control Myths
Who should NOT take BCP's  
Out of 100 women using Combination or Progestin Only BCP's
Typical Use: 8 Women Become Pregnant icon representing 5 pregnant women
Perfect Use: 1 or Less Women Become Pregnant icon representing 1 pregnant woman

What are birth control pills and how do they work?

Birth control pills (also called oral contraceptive pills and the “Pill”) are a type of female hormonal birth control method and are very effective at preventing pregnancy. The Pills are small tablets that you swallow each day. Most pills contain two types of synthetic (man-made) female hormones; estrogen and a progestin. These are similar to the estrogen and progesterone normally made by the ovaries. These pills are called “combination oral contraceptives”, and there are many different kinds.


The hormones in the pills prevent pregnancy by suppressing your pituitary gland, which stops the development and release of the egg in the ovary (ovulation). The progestin also helps to prevent the sperm from reaching the egg and changes the lining of the uterus.


Another type of pill contains only one hormone (progestin) and is called either the “progestin-only pill”, or the “mini-Pill”. It works by stopping ovulation and by helping to prevent the male's sperm from reaching the egg.


Female Reproductive Anatomy


Which birth control pill should I take?

First, talk with your health care provider about whether the Pill is right for you. If it is, discuss which pill and what dosage is best for you.


The combined pill with both estrogen and progestin is slightly more effective than the progestin-only pill. However, some women can't take estrogen, so it's better for them to take the progestin-only pill.


How effective is the Pill at preventing pregnancy?

The Pill is very effective if you take it exactly as you are supposed to - one pill a day, taken at the same time each day. You should also use back-up contraception such as condoms if you have diarrhea or vomiting, or are taking a medication that could change the effectiveness of the birth control pill. Using condoms is always important to lessen your chances of getting an STI.


If you take the Pill at the same time every day (perfect use), it's more than 99% effective. This means that if 100 women take the combination pill every day, less than 1 woman will become pregnant in a year.


Although it's obvious that the Pill is most effective against pregnancy when it's taken at the same time every day, perfect use can be difficult for both teens and adults. That's why it's often considered 92% effective. This means that if 100 women use the Pill, but don't take it perfectly, 8 or more women will become pregnant in a year.


    Next: Side Effects


Written and reviewed by the CYWH Staff at Boston Children's Hospital

Updated: 7/25/2013


Related Guides:


Did you know that over 700,000 teen girls become pregnant in the United States each year? Dealing with a pregnancy is one of the most important decisions a teen may need to make...


Emergency Contraception

Emergency contraception (EC) is a backup method of birth control for preventing pregnancy after unprotected sex...

Search Our Site
Center for Young Women's Health Center for Young Women's Health Boston Children's Hospital Boston Children's Hospital
Photo of Peer Leaders Meet Our Peers
15 Years!