What Is the Plan B Pill and What Does It Do?

Plan B, sometimes called the “morning-after pill” is one of the most commonly known forms of emergency contraception. However, there is often much confusion about how it works, how it differs from regular birth control pills, and what happens if fertilization has already occurred.

What is Plan B?

Plan B One-Step contains 1.5 milligrams of levonorgestrel, a synthetic form of the hormone progestin. Levonorgestrel has been used in hormonal contraceptives for decades. Unlike regular birth control, which is taken on a continuing schedule, Plan B is intended for occasional emergency use after unprotected sex or contraceptive failure.

How does Plan B work?

Like regular birth control, the primary mechanism of Plan B is delaying or preventing ovulation. During a normal menstrual cycle, hormonal changes eventually cause an egg to be released from an ovary. If sperm are present around the time of ovulation, fertilization can occur.

Levonorgestrel works primarily by interfering with the hormonal process that leads to ovulation. When taken before ovulation, it can delay or prevent the release of an egg, reducing the possibility that sperm and egg will meet.

Timing is therefore extremely important. Research has found that levonorgestrel emergency contraception is effective when taken before ovulation but is not effective after ovulation has already occurred.

What about fertilization and implantation?

This is where some of the confusion surrounding Plan B comes from. Older scientific studies and previous product labeling proposed several possible mechanisms, including effects on sperm or egg transport and changes to the uterine lining that might interfere with implantation. Older Plan B labeling specifically stated that the medication may prevent implantation.

However, in its 2022 scientific review, the FDA concluded that the primary mechanism of the Plan B pill is ovulation inhibition or delay. Their studies also concluded that levonorgestrel does not affect the uterine lining or implantation. 

Therefore, according to the FDA’s current research, Plan B is not considered to work by preventing implantation of a fertilized egg. Its effectiveness depends primarily on whether ovulation can still be delayed or prevented.

Also, according to the FDA, Plan B does not disrupt an established pregnancy. If pregnancy has already occurred, Plan B will not terminate it. In other words, Plan B is different from the abortion pill. Plan B is intended to prevent pregnancy rather than end an existing pregnancy.

How is Plan B different from regular birth control pills?

Plan B and many regular oral contraceptives actually use some of the same types of hormones, but they are used differently and at different doses.

Regular birth control pills are taken on a continuing schedule to prevent pregnancy over time. Depending on the type of pill, they can prevent ovulation, thicken cervical mucus, and produce other changes that make fertilization less likely. Combined pills contain estrogen and progestin, while progestin-only pills contain a progestin hormone.

Plan B contains a single 1.5-milligram dose of levonorgestrel and is designed to provide an emergency intervention after unprotected sex or contraceptive failure. Because it primarily works by delaying ovulation, it must be taken before ovulation in order to be effective.

In simple terms:

Regular oral contraception: taken routinely to prevent pregnancy over an ongoing period.

Plan B: taken after unprotected sex to reduce the chance of pregnancy, primarily by delaying or preventing ovulation.

What are the side effects?

Plan B is generally well tolerated, although temporary side effects can occur. These may include changes in the timing or amount of the next menstrual period, spotting, nausea, abdominal pain, headache, dizziness, fatigue, or breast tenderness.

In Summary

Plan B is an emergency contraceptive containing levonorgestrel. Its primary mechanism is to delay or prevent ovulation. It is most effective when taken before ovulation and is not intended to terminate an established pregnancy or work as an abortion pill.

While older studies concluded that Plan B can prevent the implantation of a fertilized egg, newer studies have concluded that levonorgestrel does not affect implantation. However, just like with regular oral contraceptives, can we know for certain that Plan B only works to prevent ovulation and has no impact on implantation? No, we cannot know that with 100% certainty.

What if you’re already wondering, “Could I be pregnant?”

If you are concerned that you may be pregnant, you don’t have to face that fear alone. A late period, a positive pregnancy test, or simply the possibility of pregnancy can bring a lot of questions and emotions.

At the Family Life Center, we believe every woman deserves truth, compassion, and support as she considers her circumstances and her options. Whether your pregnancy was planned or unexpected, you don’t have to walk through it alone.

If you have questions about pregnancy, pregnancy testing, or the support that is available, the Family Life Center is here to help.