How does the morning-after pill work?

Oral emergency contraceptives mainly prevent or delay the release of an egg, so that fertilization does not occur; they are not effective once implantation has taken place and do not disrupt an established pregnancy.

Which emergency contraceptive method is most effective?

The copper intrauterine device is the most effective form of emergency contraception, with a lower failure rate than the oral options levonorgestrel and ulipristal acetate.

Emergency Contraception

Emergency contraception is the use of a method after unprotected intercourse to prevent pregnancy, before implantation occurs. The main options are oral levonorgestrel, the selective progesterone-receptor modulator ulipristal acetate, and the copper intrauterine device, which is the most effective emergency method.

Definition

Emergency contraception is a postcoital method, pharmacological or device-based, used after unprotected or inadequately protected intercourse to prevent pregnancy primarily by inhibiting or delaying ovulation, or in the case of the copper device by preventing fertilization and implantation.

Scope

This entry covers the principal emergency contraceptive options, their mechanism of delaying or inhibiting ovulation, the comparative effectiveness of oral agents, and the role of the copper intrauterine device. It is a reference overview of how these methods work and how their effectiveness is studied, not dosing or treatment guidance.

Core questions

How do oral emergency contraceptives prevent pregnancy after intercourse?
How do levonorgestrel and ulipristal acetate compare in effectiveness?
Why is the copper intrauterine device the most effective emergency method?

Key concepts

Levonorgestrel emergency contraception
Ulipristal acetate
Selective progesterone-receptor modulator
Ovulation inhibition and delay
Copper intrauterine device for emergency use
Postcoital window of effectiveness
Pre-fertilization mechanism

Mechanisms

Gemzell-Danielsson (2010) reviews evidence that oral emergency contraceptives act principally by delaying or inhibiting ovulation, with levonorgestrel effective when taken before the luteinizing-hormone surge and ulipristal acetate, a selective progesterone-receptor modulator, able to postpone ovulation even after the surge has begun, giving it a wider effective window. The copper intrauterine device prevents fertilization through its copper-induced effects on sperm and can also prevent implantation when inserted after intercourse. These mechanisms act before or around the time of fertilization rather than disrupting an established pregnancy.

Clinical relevance

Emergency contraception provides a time-limited opportunity to prevent pregnancy after unprotected intercourse, and the choice among methods depends on the interval since intercourse and access. This entry describes the methods and the comparative evidence at a reference level; it does not provide dosing instructions or individual treatment advice, and decisions should be made with a qualified clinician.

Epidemiology

The randomised non-inferiority trial and meta-analysis by Glasier and colleagues (2010) found that ulipristal acetate was at least as effective as levonorgestrel for emergency contraception and retained effectiveness over a longer interval after intercourse. Trussell (2011) and related reviews note that oral methods become less effective the later they are taken, whereas the copper intrauterine device has the lowest failure rate of all emergency options.

History

Postcoital contraception developed from high-dose oestrogen regimens and the combined Yuzpe method to dedicated levonorgestrel-only products, which became widely available over the counter in many countries. The later introduction of ulipristal acetate extended the effective postcoital window, and recognition of the copper intrauterine device as the most effective emergency option broadened the available choices.

Debates

Do oral emergency contraceptives act after fertilization?: Mechanistic reviews indicate that levonorgestrel and ulipristal acetate work mainly by inhibiting or delaying ovulation rather than by preventing implantation of a fertilized egg, a point that has been debated in scientific and public discussion of how these methods are characterised.

Seminal works

glasier-2010
gemzell-danielsson-2010

Frequently asked questions

How does the morning-after pill work?: Oral emergency contraceptives mainly prevent or delay the release of an egg, so that fertilization does not occur; they are not effective once implantation has taken place and do not disrupt an established pregnancy.
Which emergency contraceptive method is most effective?: The copper intrauterine device is the most effective form of emergency contraception, with a lower failure rate than the oral options levonorgestrel and ulipristal acetate.