Jun. 15, 2012

Emergency Contraception: How It Works (How It Doesn’t)

by James Trussell and Kelly Cleland

Click to enlarge images
Emergency contraception (EC) offers women a last chance to prevent pregnancy after unprotected sex, whether a contraceptive method was not used (or was not used correctly), the method fails (such as a condom breaking or slipping), or in cases of sexual assault. Recently, there has been quite a bit of public discussion about emergency contraception, including an article in the New York Times. The intense focus on how the method works seems unique to EC; it is difficult to name any other contraceptive method that has inspired so much discussion around technical biological issues.
First, the basics on options for emergency contraception. Often, the term “emergency contraception” is used to refer to the levonorgestrel pill (1.5 mg), which is marketed as Plan B One-Step, Next Choice, and Levonorgestrel Pills in the United States and is available in over 140 countries (check Princeton’s Emergency Contraception website for a database of EC pills available worldwide). But this is not the only EC product available; the oldest regimen for EC pills is a combination of levonorgestrel and estrogen. In some places, where a dedicated EC product is unavailable, too expensive, or difficult to get, women take larger doses of regular birth control pills after unprotected sex (click here for a chart listing how to dose different brands of birth control pills for EC). This is not the most effective treatment, and the side effects can be uncomfortable; but for women in some parts of the world, it is the only option available. A newer pill containing the antiprogestin ulipristal acetate arrived on the European and US markets starting in 2009 (sold as ellaOne or ella). Ulipristal acetate is effective closer to the time of ovulation than levonorgestrel, and maintains its effectiveness up to 5 days after unprotected sex (when the efficacy of levonorgestrel declines). Despite these advantages, ella is not yet widely used in the US; unlike levonorgestrel pills, which are available without prescription to women and men aged 17 and older, ella is prescription-only for women of any age. In a few countries (China, Vietnam, Russia and Armenia), the antiprogestin mifepristone in very small doses is available as an emergency contraceptive. Finally, the most effective (but least-used) emergency contraceptive option is the copper IUD, which can be inserted up to 5 days after unprotected sex to prevent pregnancy, and can be left in place to provide at least 10 years of excellent contraceptive protection. (For a complete and current review of the academic literature about emergency contraception, click here.)
How do emergency contraceptives work? The idea of contraception used after sex seems counterintuitive. But the basic facts of reproductive biology allow key opportunities to interfere with the reproductive process after sex has occurred, but before pregnancy begins. Sperm can survive in the woman’s body for 5 days after sex, fertilization must occur within 12 to 24 hours of ovulation (which occurs about 14 days before the next period would have come), and implantation (according to federal medical definitions, pregnancy begins with implantation) does not occur until about 7 to 14 days later. Thus, the probability of pregnancy resulting from any particular sex act is quite limited, varying from zero during the beginning of the menstrual cycle to around 30% right before ovulation. In a typical menstrual cycle, a complex interplay of hormones prepares the body for pregnancy. A critical element of the cycle is the release of luteinizing hormone (LH) over a 24 to 48 hour period; this LH surge stimulates the ovary to release an egg. EC pills effectively inhibit or delay the release of the egg if taken at the right time during the cycle. While levonorgestrel EC pills are effective only if taken before the LH surge, ulipristal acetate can still work after the LH surge has begun (but is no longer effective once LH reaches its peak concentration). This additional window of effectiveness may be important for individuals who have had unprotected sex, as a woman who has sex just prior to ovulation – when the LH surge is underway – is at the highest risk of pregnancy.
In addition to the available biological evidence, clinical research provides important insights about how EC works. The newest and most reliable evidence comes from two recent studies (published in 2007 and 2011) in which women who came to clinics for EC were monitored to assess each woman’s menstrual cycle day. Among women who took EC before ovulation, none became pregnant. The women who took EC on the day of ovulation or after became pregnant at the rate that would be expected if they hadn’t used any contraception. This provides compelling evidence that levonorgestrel EC works by inhibiting or delaying ovulation, but is ineffective after ovulation has already occurred (and therefore would not be effective in preventing the implantation of a fertilized egg). Although parallel studies have not been conducted for EC containing ulipristal acetate and mifepristone, there is no evidence that, at the doses used for EC, these methods would effectively prevent implantation. There is some evidence that ulipristal acetate can produce changes in the uterine lining, but whether these changes would impair the implantation of a fertilized egg is unknown.
If emergency contraceptive pills were effective at preventing implantation of a fertilized egg, their failure rates would surely be lower than they are. Levonorgestrel EC fails about 2.2% of the time, and the failure rate for ulipristal acetate is around 1.4%. The copper IUD is the most effective emergency contraceptive method by far, with a failure rate of less than one per thousand; the extremely high efficacy of the IUD used as EC suggests that it does have the ability to inhibit implantation. IUDs can be difficult to access due to significant logistical and financial barriers, and EC pills are generally far easier to obtain and use. Women may very well prefer a method that has the potential to work after ovulation because it would be so effective, and for those women, the development of an emergency contraceptive pill that is effective in preventing implantation could be of tremendous value.
For more about emergency contraception, visit www.not-2-late.com; for more about how emergency contraception works, click here
James Trussell is Professor of Economics and Public Affairs and Faculty Associate of the Office of Population Research at Princeton University.  He is the author or co-author of more than 300 scientific publications, primarily in the area of reproductive health.   His recent research has been focused in three areas: emergency contraception, contraceptive failure, and the cost-effectiveness of contraception. He has actively promoted making emergency contraception more widely available as an important step in helping women reduce their risk of unintended pregnancy; in addition to his research on this topic, he maintains an emergency contraception Web site not-2-late.com.
Kelly Cleland is a staff researcher in the Office of Population Research at Princeton University and is the Executive Director of the American Society for Emergency Contraception. Kelly’s work focuses on reproductive health and contraception, particularly emergency contraception.


About James Trussell and Kelly Cleland

James Trussell and Kelly Cleland are reproductive health experts from Princeton University.

The views expressed are those of the author and are not necessarily those of Science Friday.
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