Emergency contraception: how does it work?

Area of Science:

• Reproductive Health
• Endocrinology
• Pharmacology

Background:

• Emergency contraception (EC) is crucial for preventing pregnancy post-unprotected intercourse.
• Current hormonal EC methods (e.g., high-dose combined oral contraceptive pills, levonorgestrel) are effective but have limitations.

Purpose of the Study:

• To elucidate the mechanisms of action for hormonal emergency contraception.
• To differentiate the effects of EC on fertilization versus implantation.

Main Methods:

• Review of existing research on hormonal EC (combined oral contraceptive pills, levonorgestrel).
• Analysis of biological data and clinical observations regarding EC timing and efficacy.
• Comparison with other EC methods like mifepristone and intrauterine devices.

Main Results:

• Hormonal EC methods inhibit the midcycle luteinizing hormone (LH) surge, delaying or preventing ovulation if administered at least 2 days prior.
• Efficacy decreases if administered closer to ovulation; ovulation still occurs if given when imminent.
• Evidence suggests hormonal EC primarily prevents fertilization, with no data supporting effects on embryo development or implantation.

Conclusions:

• Hormonal emergency contraception's primary mechanism is preventing fertilization by inhibiting ovulation.
• Levonorgestrel does not impair embryo development or prevent implantation.
• Mifepristone and intrauterine devices represent alternative EC methods with potential to inhibit implantation.