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How does Emergency Contraception Work?

Mary McMahon
Updated May 17, 2024
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Emergency contraception is a type of birth control which is taken within five days of unprotected sexual activity to prevent pregnancy, although the sooner it is taken, the more effective it will be. Emergency contraception is sometimes referred to as “The Day After Pill,” or “Plan B,” the brand name of a popular form of progestin-only emergency contraceptive pill. While women would be advised to have a course of emergency contraception on hand, they should not rely upon it as a form of birth control, and they should also be aware that it will not protect them from infection with sexually transmitted diseases (STDs).

Emergency contraception is not a medical abortion pill. It works by preventing an egg from leaving the ovary, inhibiting the ability of sperm to penetrate an egg, or by making implantation of the egg in the uterus impossible. Conventional birth control pills also work in this way, although they do so routinely with a lower dose of hormones rather than all at once with a high dosage, which can cause nausea, vomiting, and feelings of general malaise.

A woman may want to consider taking emergency contraception if she has had unprotected sexual activity, missed or skipped pills, forgotten to insert a ring or apply her patch, or experienced forced sexual activity. Ideally, emergency contraception should be taken in the form of a progestin-only pill, such as Plan B, because it has a higher rate of effectiveness. If emergency contraception is not readily available, it is possible to duplicate it by using high doses of regular birth control pills, under the advice of a nurse or physician.

Many patients who have used hormonal emergency contraception complain of nausea and vomiting, and patients may want to consider taking an anti-nausea pill along with their emergency contraception, or inserting the pills vaginally, allowing the hormones to be absorbed directly. There is also a non-hormonal emergency contraception option; the copper intra-uterine device (IUD) which can be left in to prevent pregnancy, or taken out at the next menstrual period. An IUD must also be inserted within five days of the incident.

No form of emergency contraception will impact an existing pregnancy, although doctors will not prescribe it to a woman who is known to be pregnant. Also, because emergency contraception does not protect against STDs, women may want to be tested for these after unprotected sex with an unknown partner. To get emergency contraception and STD testing, women can seek out a local public health or planned parenthood clinic, or visit a private physician.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a WiseGeek researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By cristopher — On Sep 03, 2016

Plan B - Morning after pill is the best emergency contraception. You have to take it under 72 hours of unprotected intercourse.

By anon305724 — On Nov 27, 2012

This treatment is not an abortion of any type, and please let's keep the facts straight. All recent studies support this position. Plan B, a form of the FDA-approved emergency contraceptive option, prevents or delays the release of an egg, thereby preventing fertilization, ergo no pregnancy.

Likewise, a 2010 study found that Ulipristal Acetate a.k.a. Ella, at certain doses, can decrease the thickness of the endometrium, but it is not clear that this would, in fact, prevent the implantation of a fertilized egg.

Therefore, I have no problem with the use of emergency contraception. As a Christian, I prefer honest discussion and not one-sided fact tampering. Exaggerating information is not the best way to support one's viewpoint.

By anon515 — On Apr 26, 2007

Unfortunately, "making implantation of the egg in the uterus impossible" is in effect causing a chemical abortion, at least to anyone that believes life begins at conception (and conception occurs at fertilization not a week later at implantation). It is of course, a fertilized egg that is prevented from implanting.

That is why this treatment is considered an abortifacient.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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