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A morning after pill failing

Illustration by Marta Pucci

Reading time: 8 min

Why the emergency contraception pill fails

Everything you need to know about the effectiveness of emergency birth control

Top things to know about emergency contraception:

  • In the USA, there are two different types of emergency contraceptive pills: one is available over the counter, the other needs a prescription.

  • One-dose emergency contraception pills prevent pregnancy between 50-100% of the time. 

  • Unpredictable ovulation, body type, and drug interactions, can reduce the effectiveness of the contraceptive pill.

  • To prevent pregnancy, it’s always better to take the emergency contraceptive pill than to do nothing at all. 

  • You can get an emergency contraceptive pill in case you need one in the future.

Most of the time, emergency contraception works, when it’s taken within the recommended window after unprotected sex. 

The two types of emergency contraception most commonly used are: emergency contraceptive pills and the copper intrauterine device (IUD) (1). Recent research suggests that some hormonal IUDs (Levonorgestrel-releasing IUDs like Mirena and Liletta) can be just as effective as the copper IUD as emergency contraception (2)

In this article, we are going to focus on emergency contraceptive pills, which are also called emergency birth control or morning-after pills—we’ll use these terms interchangeably. In some cases, combined oral contraceptives (“the pill”) can also be used as emergency birth control.

The effectiveness of the morning-after pill can be hard to study, because it can be difficult to know when people are ovulating, when they had sex and what day they are on in their cycle (3,4). Based on the data scientists have, one-dose emergency contraception pills are effective at preventing pregnancy about 50-100% of the time (4,5). The range is so wide because people don’t get pregnant every single time they have unprotected sex, and there are lots of different factors which lead to conception

What are different types of morning-after pill?

Progestin emergency contraception (also known as levonorgestrel

Available over-the-counter in the USA

In the USA, the progestin-only emergency contraception pill can be purchased in pharmacies by anyone of any age without a prescription (1). You should take it as soon as you can after unprotected sex, but it’s most effective if taken within 72 hours (1). You can take it for up to five days after sex. If you take it between three and five days, it might not be as effective, but is still worth taking (1).

It works by delaying the release of the egg from the ovary (ovulation), which prevents the sperm  from fertilizing it (6).

Antiprogestin emergency contraception pill (also known as ulipristal acetate)

Available by prescription in the USA

Antiprogestin emergency contraception pills are the most effective form of emergency contraceptive pill (1) and available only by prescription in the USA. This pill should be taken as soon as possible after unprotected sex (7), but you can take it up to five days later (1).

This emergency contraception pill changes the way progesterone works in your body (1). It works by preventing or slowing ovulation (1). When ovulation is postponed or halted, there is no egg for sperm to fertilize, and pregnancy is prevented.

What reduces the effectiveness of emergency birth control?

Emergency contraception can be effective at preventing pregnancy, but sometimes people take it and still become pregnant. Here are the three main reasons why this happens. 

1. Ovulation happened before you took the pill

Emergency birth control is all about timing. It's recommended that you take the pill as soon as you can—if you wait too long, you might miss the window during which the pill can be effective.

If you take it right after sex, it can prevent you from ovulating if you haven’t started already (1). If you have sex at the time you ovulate or after you ovulate, your emergency contraception pill won’t be effective (8). 

If you have unprotected sex again after you take the pill in the same cycle, it might not work (8). Use a barrier method like a condom if you have sex. 

2. The morning-after pill is more effective for some body types

Healthcare providers use the Body Mass Index (BMI) scale to group people into generic categories based on height and weight (9). (Remember that while BMI is often used as a screening tool, a higher BMI doesn’t mean someone is unhealthy.)

Current studies show that emergency contraceptive pills are less effective for people who have a BMI of 30 or higher (8,10). 

People who have a BMI of 30 or higher who take emergency contraception might become pregnant more frequently than those with a BMI of 25 or less (8,10). This doesn’t mean if your BMI is 30 or higher that you shouldn’t take emergency contraception. Note: BMI doesn’t consider a person’s age, sex-assigned at birth, muscle mass, or other health factors like food intake or exercise. It also doesn’t differentiate between lean body mass and fat mass, which means people like athletes with low body fat can have a high BMI (11). 

If you have a BMI of 25 or higher, you might want to call your healthcare provider for a prescription for the antiprogestin emergency contraceptive pill (8). It might offer better prevention of pregnancy for your body type (8). If you have a BMI of 30 or higher, you might want to discuss options with your healthcare provider including an IUD for emergency contraception (1,8). If you do not have access to a healthcare provider, taking an over-the-counter progestin emergency contraceptive pill is better than taking nothing. 

If you are considering an IUD as emergency birth control, a healthcare provider must insert it within 5 days after you have unprotected sex (1). You should call your healthcare provider, or nearest Planned Parenthood health center as soon as you realize you need emergency contraception. It can be difficult to get an appointment for an IUD insertion on short notice, so call as quickly as possible.

If you can’t get an appointment to have an IUD put in within 5 days after you have unprotected sex, take the morning-after pill as soon as possible. Keep in mind that in order to prevent pregnancy, it’s always better to take the morning-after pill than to do nothing at all (10).

3. You’re taking a medication that interacts with emergency contraception

Some drugs and herbal products can cause emergency birth control pills to be less effective (7, 12). See the table below. 

Be sure to double check the emergency contraceptive package information for more detailed information. 

Important: Do not stop taking any prescription medication without first consulting your healthcare provider. Doing so can cause severe side effects. 

Hormonal birth control can also cause interactions with the antiprogestin emergency contraceptive pill. We’ll talk more about taking hormonal birth control after the emergency contraceptive pill, below.

What to do after you take emergency birth control

Progestin emergency contraception (also known as levonorgestrel) 

Available over-the-counter in the USA

After you take the progestin-only emergency contraception pill, use a barrier method like external or internal condoms or don't have sex for least seven days (1). You can continue or start taking hormonal birth control (like the pill, patch, shot, or ring), start straight away (1).

Antiprogestin emergency contraception pill (also known as ulipristal acetate)

Available by prescription in the USA

If you take the antiprogestin emergency contraceptive pill, use barrier methods or avoid having sex until your next period. If you want to start (or continue) taking hormonal birth control, wait until five days after taking the emergency contraceptive pill (1, 13). Hormonal birth control can cause the antiprogestin emergency contraceptive pill to be less effective if taken at the same time (1, 13). 

After following these precautions, you can go about living your life. You don't need any follow-up tests or procedures (1). You don't even need to tell your healthcare provider (unless you want to) (1). If your period is over a week late, take a pregnancy test to be sure you aren’t pregnant (1).

What happens if you take emergency contraception and get pregnant? 

Emergency contraceptive pills won't harm the pregnancy or the fetus (1). If you are considering abortion, reach out to a healthcare provider as soon as possible. 

Signs that your emergency contraception pill didn’t work

Taking an emergency contraception pill can delay your period by one week (1). If you think you are pregnant, you should take a home pregnancy test and call your healthcare provider as soon as possible.

Possible early signs of pregnancy (14,15): 

  • Missed period

  • Feeling very tired

  • Headache

  • Feeling lightheaded or faint

  • Food aversions or cravings

  • Sensitivities to smells

  • Swollen or tender breasts

  • Pelvic, back, or leg pain

  • Bloating

  • Light spotting and/or cramping

  • Nausea / vomiting 

  • Needing to pee often

  • Constipation

  • Mood swings

Getting the morning-after pill before you need it

It is a good idea to stock up on an emergency contraceptive pill in case you need one in the future. You can ask your healthcare provider for a "just-in-case" prescription or grab an over-the-counter pill from the pharmacy to keep at home.

Don't forget—the emergency contraceptive pill, like hormonal birth control, does not prevent sexually transmitted infections (STIs). If you think you might be at risk for STIs, always use a barrier method like a condom when you have sex and talk to your healthcare provider about getting an STI screening.

This article was originally published September 18th 2019.

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