Signs that Plan B didn't work: Why it happens & what to do

At the same time, it may ease your mind to know that emergency contraception is highly effective — Plan B specifically is about 87% effective when used as directed [1]. In this article, we’ll go over what you may want to look out for in terms of signs that plan B didn’t work, and answer some common questions about emergency contraception including “does Plan B work during ovulation?” and more.

What are the signs Plan B did not work?

While it’s very tempting to try to use signals from your body to tell whether or not Plan B successfully prevented pregnancy, the only definitive way to confirm whether or not you’re pregnant is with a pregnancy test. Most pregnancy tests advise waiting until the first day of your missed period to take the test.

This is because the path to pregnancy can take quite a few days, or even weeks. For pregnancy to happen, a sperm cell needs to fertilize an egg cell during ovulation, that fertilized egg cell needs to implant into the uterine wall, and only then does the pregnancy hormone human chorionic gonadotropin (hCG) begin to form in your body [2]. 

Rising hCG is the only way to be sure that you’re pregnant, which is why pregnancy tests work by detecting that hormone in your urine. Those hormone levels don’t usually start to rise until about 12 days after you’ve ovulated. That’s why it’s recommended to wait until the first day of your missed period — that’s enough time for hCG to become detectable [3]. Many common symptoms of early pregnancy are attributed to those changing hormone levels. These symptoms include:

• Nausea and vomiting
• Fatigue
• Tender breasts
• More frequent urination
• Abnormal smells, tastes, and cravings [4]

On the other hand, many of the side effects of Plan B (outlined below) are quite similar:

• Nausea
• Fatigue
• Breast pain
• Headache
• Digestive issues [5]

This is why it’s not advised to use your physical symptoms as signs that Plan B didn’t work — they’re too easily confused with Plan B’s most common side effects. Furthermore, based on your circumstances and medical history, you may experience some, all, or none of these side effects. 

Keep the pregnancy test handy for the first day of your missed period for the most certain way to know if Plan B prevented pregnancy. We know this is easier said than done, and this waiting period can be highly tense and frustrating. Again, a gentle reminder that Plan B is highly effective when used as directed, but there’s a small chance it may also delay your period, so taking that pregnancy test is your best bet [5].

What is emergency contraception?

Let’s take a step back and go over exactly why emergency contraception is so effective. Emergency contraception methods are measures you can take to prevent pregnancy after a primary birth control method failed. Some examples of when this would be a good course of action include if a condom broke or slipped off during intercourse, if you’re on the pill and forgot to take multiple pills in a row, or if you didn’t use any form of contraception during intercourse and still want to prevent pregnancy [6]. Although there may be some social stigma around it, about one in four women in the U.S. between the ages of 22 and 49 have used emergency contraception [7].

It comes in the form of pills (like Plan B) or the copper intrauterine device (IUD) — more on that later. For pills specifically, they should not be used as a long term, primary birth control method because they won’t be as effective as other forms of birth control, and there may be more prevalent side effects [6]. Learn more about how many times you can take Plan B here.

How does emergency contraception work?

Below, we’ll share more details about the different forms of emergency contraception and how they work. But first, we want to note that there is no form of emergency contraception that will terminate a pregnancy, nor will it harm the embryo if you’re already pregnant [8]. This is why you’ll want to take emergency contraception pills as soon as possible in the recommended timeline — you’ll have more control over your choices going forward.

Progestin emergency contraception pill (levonorgestrel or “Plan B”)

We’ll start with one of the most widely used forms of contraception in the U.S., which is the progestin-only pill (also commonly referred to as Plan B, its brand name). Plan B contains a high dose of levonorgestrel, a specific type of progestin [9]. Progestin is a synthetic hormone found in other types of hormonal contraceptives like birth control pills. Plan B is FDA approved for all age groups, widely accessible at local pharmacies, and doesn’t require a prescription [5].

When taken as an emergency contraceptive, it suppresses luteinizing hormone, thus stopping or delaying ovulation [10]. By stopping ovulation, no egg cell is released for a sperm cell to fertilize, thus making pregnancy impossible. By delaying ovulation, sperm cells will likely disintegrate before they have a chance to fertilize the egg cell released at ovulation [6]. 

But remember — sperm can live for up to five days in the female reproductive tract. This is part of why Plan B is most effective when taken within three days of unprotected intercourse, and only moderately effective if taken within five days. There is more of a chance that sperm has already fertilized an ovulated egg during this window [6].

With that in mind, if you’re asking yourself “does Plan B work during ovulation?” the answer is a bit more complicated than a yes or no. Plan B actually works by affecting ovulation in order to prevent pregnancy. If you’ve already ovulated, it won’t be as effective.

Antiprogestin emergency contraception pill (ulipristal acetate or ella)

Another type of emergency contraception pill is the antiprogestin pill, sold under the brand name ella. Its chemical compound, ulipristal acetate, is thought to affect how the hormone progesterone works in the body, and it also prevents pregnancy by delaying or stopping ovulation. It’s slightly more effective than Plan B for a longer period of time (up to five days after unprotected intercourse), but it’s generally more expensive than Plan B and only available via a prescription from your healthcare provider [6, 11].

Copper intrauterine device (IUD)

The copper IUD is the most effective type of emergency contraception. It prevents pregnancy by making sperm less capable of fertilizing an egg cell and it can prevent implantation [6]. It’s a small T-shaped device that is placed into your uterus by a healthcare professional. Once inside, it releases copper into the environment. Copper is toxic to sperm, making it difficult to reach the egg. It can be quite uncomfortable to get one inserted, and you may feel some pain after the procedure, but you’ll be protected immediately from pregnancy. If you get it within five days of another birth control method failing, it’s the most effective emergency contraceptive and you’re protected for the next five to ten years [12].

A note on Plan B effectiveness and BMI

Recently, there has been chatter online about evidence suggesting that the effectiveness of Plan B declines as BMI (body mass index) goes up, meaning that it may not be as effective for you if you have a BMI of over 25, which is classified as overweight [13].

However, BMI is a controversial health metric, the overall evidence for this trend is inconclusive, and guidelines generally have not changed — if you’d like to take Plan B, it’s recommended to take it as soon as possible after unprotected sex and before ovulation to prevent pregnancy no matter what you weigh. Timing is the primary factor when it comes to the efficacy of Plan B, not necessarily weight. The sooner you take it, the better it works [1].

The copper IUD does not seem to be affected by BMI, so you may want to explore those as alternatives if you’d like more peace of mind [13]. We always recommend speaking with a healthcare professional about your options to help make the best decisions for your body.

Plan B side effects

Although Plan B is highly effective at preventing pregnancy when used as directed, the manufacturer does warn of some potential side effects including:

• Changes to your period (it might be lighter, heavier, early, or late)
• Nausea and vomiting
• Cramps in the lower abdomen
• Tiredness
• Headache
• Dizziness
• Breast tenderness

Again, depending on your unique circumstances, you may experience some, all, or none of these in the days after you take Plan B. However, if you vomit within a few hours of taking either Plan B or ella, reach out to your healthcare provider as soon as possible to see if you should repeat the dose [1].

Can Plan B make your period late or cause bleeding?

Yes, Plan B can affect your period by making it lighter or heavier than you’re used to, and it may be delayed during the next cycle. Some people also experience light spotting or bleeding prior to their period after taking Plan B. If your period is late after taking plan B, it’s best to take a pregnancy test [1]. If your period is late and the pregnancy test is negative, some possible explanations here and reach out to your doctor for more information.

Can you have sex after taking Plan B?

Yes, you can have sex after taking Plan B, but if you’re trying to prevent pregnancy, it’s strongly recommended that you rely on your primary birth control method when you do decide to have sex again. You can take Plan B alongside any birth control method you’ve been using (explore some non-hormonal birth control methods here). And finally, if you’d like to get pregnant in the future, know that Plan B will not affect your fertility moving forward after delaying ovulation for this specific cycle [1].

Can you drink alcohol after taking Plan B?

Yes, in general it is generally safe to drink alcohol while on any type of birth control, including emergency contraception [14].

What are the chances of pregnancy after taking Plan B?

According to the manufacturer, Plan B is about 87% effective at preventing pregnancy, meaning that when it’s taken as directed, about seven out of every eight people successfully prevent pregnancy [1]. The FDA also cautions that certain medications and herbs can interact with Plan B and decrease its effectiveness. These include:

• Barbiturates
• Bosentan
• Carbamazepine
• Felbamate
• Griseofulvin
• Oxcarbazepine
• Phenytoin
• Rifampin
• St. John’s wort
• Topiramate [5]

What to do if Plan B failed?

If you have concerns that Plan B failed, you can always reach out to your healthcare provider for help. They’ll be able to give you the best recommendations considering your unique circumstances, and can advise you on your options for next steps. Remember, the only sure sign that Plan B didn’t work is a positive pregnancy test at the time of your missed period. If you get a positive result, reach out to your healthcare provider for further testing and advice.

Get to know your body better with Natural Cycles

If you’re sexually active and don’t want to get pregnant, we highly recommend finding an effective primary birth control that works for you and your lifestyle so that emergency contraception can be saved for true emergencies. If you’d like to learn more about your fertility and non-hormonal birth control options, why not see if Natural Cycles is a fit? It’s the first FDA Cleared birth control app, giving you peace of mind on your fertility journey. You can also log use of emergency birth control right in the app, and the algorithm can then adapt to any changes in your cycle. See if NC° Birth Control is a good fit for you today.