How Does Emergency Contraception Work?

How does emergency contraception work? Glad you asked. Let's take a look at the available evidence.

First, some [oversimplified] basics.

What is emergency contraception (EC)?

EC is a group of birth control methods; the postcoital one, to be more exact. When you use a method from another group--an implant, the condom, Persona, sterilization--you use the method before, or during intercourse. When you use an EC method, you use it after you've already had sex. Again, what distinguishes the EC group from the other birth control groups is the timing of use. EC is postcoital (after sex) birth control; all the other methods are used before, or during intercourse:

An emergency contraceptive method is any drug or device used after an unprotected intercourse to prevent an unwanted pregnancy. It is, thus, a method that is used after coitus [sex] but before pregnancy occurs and is, therefore, not an abortifacient.

What are some of the methods in the EC group?

There are two types of EC methods: pills, and intrauterine devices (IUDs). The EC pills (ECPs) can be further subdivided into: high-dose estrogen; estrogen/progesterone (the combination Pill; Yuzpe regimen); progestin-only (Plan B); mifepristone (Mifeprex; RU486); etc. [Our discussion focuses on the pill methods.]:

Trials on emergency contraception were first described in the 1930s using high doses of stilbestrol. In the late 1970s, Yuzpe introduced a regimen consisting of 0.1 mg ethinylestradiol [estrogen] and 0.5 mg levonorgestrel [progestin], given within 72 h of the intercourse and repeated after 12 h.

The "Yuzpe" regimen basically entails using your regular brand of combination birth control pills on a modified, emergency contraception, regimen.

LNG [levonorgestrel] alone [Plan B] has been widely used in contraceptive formulations and has been used successfully for postcoital contraception as well. Higher efficacy and fewer side effects have been recorded when compared to the earlier method using an estrogen-gestagen combination [the "Yuzpe" regimen, or combination Pill]. A large multicenter study comparing the two regimens have shown LNG to prevent about 85% of the expected pregnancies if no other method was used.

Recently, treatment with levonorgestrel-only [Plan B] and mifepristone [Mifeprex, RU486] has emerged as the most effective hormonal methods with very low side effects.

An aside about mifepristone [Mifeprex, RU486]: Its mechanism of action is dose-, and regimen-dependent. When used as a single low dose ECP (older dosage 600 mg; current one, 10 mg), it acts as birth control and prevents pregnancy.

Theoretically, how should EC work to prevent a pregnancy?

Unprotected intercourse may occur at any time during the menstrual cycle but it is only during a limited period, from about 5 days before to 1 day after ovulation* that it may result in a pregnancy. To be effective, postcoital treatment could theoretically target one or several of the following events: sperm transport and function, follicular development [egg maturation, inside the ovary], ovulation, fertilization, embryo development and transport, implantation or the corpus luteum [what's left of the follicle, after the egg has been expelled; essential for early pregnancy support].

*[Recall that sperm may survive in the female reproductive tract for up to five days, and the mature egg may be fertilized over a 24-hour period.]

Second, let's look at what the studies tell us about ECP's mechanism of action.

How does EC work, in real life?

Human and animal studies have shown effects at several stages of the reproductive cycle: ovulation, fertilization, gamete transport, function of the corpus luteum and implantation.

The evidence shows ECPs work by preventing ovulation. There is no direct evidence they prevent fertilization. There's also no direct evidence they prevent implantation in humans.

Looking at ECPs' effect on:

Ovulation (release of mature egg from the ovary)

- inhibit or delay [established, main mechanism of action]

Fertilization (union of egg and sperm)

- no direct evidence for prevention [effect cannot be ruled out]

Gamete Transport

- no evidence of impaired transport in humans

Effects on the Function of the Corpus Luteum

- all pills disrupt this phase, however it is not known whether such changes are incompatible with pregnancy. [Better evidence for mifepristone vs. the other types of ECPs.]

Implantation (burrowing of a fertilized egg into uterus)

- ...although the postovulatory administration of estrogen or levonorgestrel inhibits implantation in some animals, evidence of similar
effects in women has been difficult to obtain. Minor changes in the histologic and biochemical features of the endometrium occur when high-dose estrogen, the estrogen/progestin combination, or danazol is administered after ovulation, but the effects may not be sufficient to inhibit implantation. In a recent morphometric study, postovulatory administration of estrogen plus progestin had only minor effects on the endometrium, and danazol had no effect.

- Mifepristone administered immediately after ovulation delays endometrial maturation

Bottom line:

...the proven mechanisms of action [of the hormones found in ECPs] consist of inhibiting or delaying ovulation.

and

An emergency contraceptive method is used after coitus but before pregnancy occurs....Recently, treatment with either 10 mg mifepristone [RU486] or 1.5 mg of levonorgestrel [~Plan B] has emerged as the most effective hormonal method for emergency contraception with very low side-effects. However, the knowledge of the mechanism of action of mifepristone and levonorgestrel in humans, when used for contraceptive purposes and especially for emergency contraception, remains incomplete.

...

When summarized, available data from studies in humans indicate that the contraceptive effects of both levonorgestrel and mifepristone, when used in single low doses for emergency contraception, involve either blockade or delay of ovulation, due to either prevention or delay of the LH surge, rather than to inhibition of implantation.

The last word goes to the NEJM (scroll to the bottom):

Use of emergency contraception is limited largely by ignorance. Although it seems likely that the estrogen/–progestin regimen works mainly by interfering with ovulation, it is nevertheless regarded by many as an abortifacient because it is taken after, rather than before, intercourse. This confusion is compounded when mifepristone is advocated for emergency contraception since, when taken after pregnancy is established, it can be and is used for the induction of abortion. The prevention of pregnancy before implantation is contraception and not abortion. Intervention within 72 hours after intercourse cannot possibly amount to abortion, because implantation is not achieved until at least seven days after ovulation and the egg is capable of being fertilized for only about 24 hours.

Technorati Tags: emergency contraception, Plan B, contraception, birth control, reproductive health, health and wellness

[Pardon the abundance of tags; I've just discovered how to use them.]

Update: More here (via ourword)

Another Update: And make sure not to miss this post on Plan B from Pharyngula.