There are plenty of reasons why you might be in need of emergency contraception, but they basically all come down to one main goal: preventing an unintended pregnancy. What’s less clear, however, is how emergency contraception actually works to prevent that pregnancy.
Before we get into the details, it’s important to realize that there are a few different options for emergency contraception. When you think about the morning-after pill you’re typically talking about two different methods: a pill with levonorgestrel (like Plan B One-Step) or a pill with ulipristal acetate (ella). But there’s also a third option for emergency contraception that’s not exactly available at your pharmacy: the copper IUD.
Now, let’s get into how each one works to prevent an unintended pregnancy.
This is how levonorgestrel-based emergency contraception works.
According to the American College of Obstetricians and Gynecologists (ACOG), the most commonly used emergency contraception in the United States is levonorgestrel, a synthetic form of the hormone progesterone. You probably know this by the brand name Plan B, though it’s technically called Plan B One-Step as it’s just one pill (as opposed to the original version of Plan B that included two pills taken 12 hours apart). Plan B One-Step and its generics are made up of a single pill containing the full 1.5-milligram dose. Levonorgestrel is available over the counter without age restrictions, according to ACOG, and is advised for use up to 72 hours after unprotected sex.
Interestingly, levonorgestrel is the same ingredient that’s in a few other methods of birth control, like certain birth control pills and the hormonal IUD. The difference, of course, is the dose.
This medication works by delaying or preventing ovulation. So, let’s say you have sex on Saturday and you have no idea if or when you’re ovulating, but you do know that you don’t want to risk an unintended pregnancy. If you ovulate around the time that you had sex, your body will release an egg, and that egg might get fertilized by the sperm that’s hanging out in your body from the sex. But what if you could delay ovulation so that there’s no egg around for the sperm to fertilize?
That’s where levonorgestrel emergency contraception comes in—if it can delay ovulation for long enough, any sperm in your system will die before there is an egg available to be fertilized, Holly Bullock, M.D., M.P.H., an ob/gyn and assistant professor of obstetrics and gynecology at the University of Arizona College of Medicine and fellow for Physicians for Reproductive Health, tells SELF.
Levonorgestrel appears to make this happen by mimicking the increased levels of progesterone that normally occur in the days after ovulation. This effectively tells your brain to decrease its production of follicle-stimulating hormone (FSH), which is responsible for developing a follicle to release an egg in ovulation. This also prevents an increase in luteinizing hormone (LH), which typically surges around day 14 of your menstrual cycle to cause ovulation. Without sufficient levels of these hormones, your body doesn’t get the message to release an egg when it usually would.
Worth noting: Some research shows that levonorgestrel may be less effective for people with high BMIs. However, the FDA has maintained that there is not enough research to require a change in how the drug is labeled or prescribed in the United States. Also, as previously reported in an in-depth SELF article on the subject, “less effective” doesn’t necessarily mean “not effective.”
The most crucial factor when it comes to effectiveness is how soon you take levonorgestrel after sex. Despite the nickname “the morning-after pill,” you should take levonorgestrel as soon as possible after unprotected sex. “Hormonal methods are more effective the sooner they are administered,” Tina Raine-Bennett, M.D., M.P.H., an ob/gyn and senior research scientist at Kaiser Permanente department of obstetrics and gynecology and division of research, tells SELF. That way, if the hormonal changes that induce ovulation are impending, the levonorgestrel has more time to stop them before an egg is released.
Since the timing of ovulation is so important here, levonorgestrel is less effective in cases where that LH surge has already begun. At that point, the release of an egg is imminent, which increases the odds of pregnancy. “If people could know exactly when they ovulate each month, it would be much easier to determine if emergency contraception is needed,” Wing Kay Fok, M.D., M.S., clinical assistant professor of obstetrics and gynecology at Stanford University, tells SELF. “But since most people’s menstrual cycles are not perfectly regular, we recommend emergency contraception after any unprotected sex, and as soon as possible.” And, because levonorgestrel just works to prevent or delay ovulation, it won’t do anything to stop an existing pregnancy.
When you do take levonorgestrel, you may experience side effects such as irregular bleeding, abdominal discomfort, fatigue, headache, nausea, and vomiting. “These side effects are generally mild and go away on their own,” Kelly Cleland, M.P.H., a research specialist at Princeton University, coordinator of the American Society for Emergency Contraception, and consulting associate at the reproductive health advocacy organization Gynuity, tells SELF. However, if you vomit within two hours of taking levonorgestrel, you may need to take another dose. Call your health care provider to ask their opinion.
This is how ulipristal acetate works to prevent pregnancy.
The other main type of morning-after pill is ulipristal acetate (ella), which is taken as a single 30-milligram pill. It also works by hampering ovulation, but in a different way than levonorgestrel.
The mechanism isn’t entirely understood, but it appears that ulipristal acetate modifies progesterone receptors in the follicle that surrounds the developing egg in the ovary, ultimately barring the release of the egg into the fallopian tubes, meaning ovulation is delayed or prevented.
Ulipristal acetate is designed to be taken within 120 hours (five days) of unprotected sex. Unlike levonorgestrel, its efficacy does not decrease significantly over that time period. “Ulipristal acetate … continues to delay follicular rupture closer to the time of ovulation,” Dr. Raine-Bennett explains. Its apparent mechanism (preventing the rupture of a follicle containing a mature egg) is capable of working even after LH has begun to rise, so it can be effective closer to the time of ovulation than levonorgestrel.
The most common side effects of ulipristal acetate are generally the same as levonorgestrel, including headache, stomach pain, wonky bleeding, and nausea. Vomiting within three hours of taking ulipristal acetate warrants a call to the doctor for the next steps.
One really important thing to know here: It can be harder to get your hands on ulipristal acetate than on levonorgestrel. Firstly, you need a prescription. “There’s really no good medical reason,” Dr. Fok says. “The best explanation is that ulipristal acetate is newer to the market.” (The FDA approved levonorgestrel as emergency contraception in 1999 and ulipristal acetate in 2010.) Another issue is that health care providers are often less familiar with ulipristal acetate than with levonorgestrel, Cleland says. If you are specifically in need of ulipristal acetate (like if it’s been four days since unprotected sex), Dr. Bullock recommends trying a resource like Planned Parenthood.
This is how the copper IUD works to prevent pregnancy.
Finally, we can’t talk about emergency contraception without discussing the copper intrauterine device (IUD). The copper IUD is a small, plastic, T-shaped tool with copper looped around the outside, which is inserted into the uterus. This is not a DIY deal; to get a copper IUD as an emergency contraceptive, you’ll need to make an appointment to have a medical provider do the insertion.
You probably know that the copper IUD can give you pregnancy prevention for up to 10 years, but it can also be used as emergency contraception if it’s inserted within five days of having unprotected sex. Weight also doesn’t impact its efficacy, Dr. Bullock adds.
The copper IUD is just as effective at preventing pregnancy on the fifth day as it is on the first. Instead of using temporary hormones to prevent or delay ovulation, the copper IUD creates a long-term toxic effect in the uterus that physically damages sperm’s ability to work properly. “The IUD releases copper ions which impair sperm function … to prevent fertilization and may also prevent implantation,” Dr. Fok explains. However, copper IUDs do not disrupt implantation of a fertilized egg that has already occurred, according to ACOG. In the medical field, implantation of a fertilized egg is viewed as the starting point of pregnancy, ACOG explains. If the copper IUD could affect implantation that had already occurred, it would be categorized as an abortion method, but it’s not.
The main points of consideration for a copper IUD are the price, insertion process, and side effects. Depending on your insurance, a copper IUD can be more than $ 1,000. (Ulipristal acetate and brand-name levonorgestrel both cost around $ 40-$ 50; generic levonorgestrel or the brand-name version with a prescription may be cheaper.) IUD insertion can also be quite painful for some people, and you might experience heavier periods and more intense cramps for at least the first three to six months, Dr. Bullock says.
However, Dr. Bullock adds that one of the great benefits of the copper IUD is that it offers long-term contraception, as it is recommended for up to 10 years. If you do want to get pregnant, you can get the IUD removed and quickly return to your age-dependent level of fertility.
A few final notes on emergency contraception:
“Emergency contraceptive pills are not as effective as ongoing contraceptive methods,” Cleland says. “If emergency contraception is the sole method [of birth control] for someone who has sex frequently, the pregnancy risk may be relatively high.”
That’s because the success of emergency contraceptive pills is so dependent on one instance of delaying or preventing ovulation. In contrast, the various mechanisms involved in long-term contraception are designed to make your body less hospitable to pregnancy in a continuous, less fallible way. Birth control may do this with estrogen to (much more reliably) suppress ovulation. There’s also progestin-only birth control to thin your uterine lining so it’s harder for a fertilized egg to implant, along with thickening your cervical mucus so sperm have a harder time moving. Some modes of birth control have both estrogen and progestin, making use of all of these mechanisms. In any case, long-term birth control is a much more effective way to avoid pregnancy than simply relying on emergency contraception.
Also remember that aside from the copper IUD, these methods work after unprotected sex, not before it. “Oral emergency contraception only works to prevent pregnancy from an act of intercourse that already happened,” Dr. Fok says. “If you have sex after taking oral emergency contraception, you are still at risk of getting pregnant.” It’s also worth noting, she says, that none of these methods protect against sexually transmitted infections.
But, listen, things happen. And, fortunately, emergency contraception exists for those moments. With a better understanding of how emergency contraception actually works, you can make an informed choice that works for your body and your life.