Birth Control Pills

Birth control pills (also called oral contraceptives or “the pill”) are used by millions of women in the United States to prevent pregnancy.
Birth control pills are made of hormones that prevent ovulation. The hormones in the pill cause changes in the cervical mucus and the lining of the uterus. The cervical mucus thickens, which blocks the sperm from entering the cervix. The lining of the uterus thins, making it less likely that a fertilized egg can attach to it. Together, these events make it very unlikely that someone taking the pill will become pregnant.

The pill is a very effective form of birth control. When women use the pill correctly, fewer than 1 in 100 will get pregnant over 1 year. But, about 8 in 100 typical users (8 percent) will become pregnant. This is because one or more pills may be missed or are not absorbed (due to vomiting, for instance).

The birth control pill is easy to use and convenient. But it does not protect against sexually transmitted diseases (STDs). If you are at risk for STDs, you should use condoms for protection.

Combination Pills
Combination birth control pills contain the hormones estrogen and progestin (a synthetic form of the hormone progesterone).

There are many different brands with different doses of hormones. This gives a woman a choice in finding a pill that is right for her.

How to Take Them
Based on convenience and advice of their doctors, different women may start taking their pills at different times. Discuss your options with your doctor.

Pills come in packs of 21, 28, or 91 pills:

  • 21 Pills — If your pack has 21 pills, take one pill at the same time each day for 21 days. Wait 7 days before starting a new pack.
  • 28 Pills — If your pack has 28 pills, take one pill at the same time each day for 28 days. The first 21 pills contain hormones. The last seven do not.
  • 91 Pills (also called continuous dosing or extended use) — You take a pill every day at the same time each day for 91 days. The first 84 pills contain hormones. The last seven do not.


The combination birth control pill has benefits in addition to preventing pregnancy. Using this type of birth control pill may reduce the risk of certain health problems:

  • Cancer of the uterus and ovary
  • Ovarian cysts
  • Pelvic infection
  • Bone loss
  • Benign breast disease
  • Symptoms of polycystic ovary syndrome
  • Anemia (iron poor blood)
  • Ectopic pregnancy

The pill also helps to keep your periods regular, lighter, and shorter and reduces menstrual cramps.

Progestin-Only Pills
Another type of birth control pill, called the progestin-only pill or the minipill, contains only progestin. The progestin-only pill is a better choice for women who have certain health problems — such as blood clots — and cannot take pills with estrogen.

How to Take Them
The minipill comes in packs of 28 pills. All the pills in the pack contain hormones. It is important not to miss a pill.

Progestin-only pills do not offer the same benefits that combination birth control pills offer. Most people who choose the progestin-only pill do so because there are reasons they should not take estrogen.

Side Effects
Depending on the type of pill, some women may have one or more of the following side effects when taking the pill:

  • Headache
  • Nausea
  • Dizziness
  • Breakthrough bleeding
  • Missed periods
  • Tender breasts
  • Depression
  • Anxiety

The pill is a good choice for women who may want to get pregnant later. It is a safe and effective way to prevent pregnancy.

This excerpt from ACOG’s Patient Education Pamphlet is provided for your information. It is not medical advice and should not be relied upon as a substitute for visiting your doctor. If you need medical care, have any questions, or wish to receive the full text of this Patient Education Pamphlet, please contact your obstetrician-gynecologist.

To ensure the information is current and accurate, ACOG titles are reviewed every 18 months.

Copyright © January 2006 The American College of Obstetricians and Gynecologists