After taking Plan B, your period could come a few days early, right on time, or up to a week late. The exact shift depends on where you were in your menstrual cycle when you took the pill. If your period is more than a week late, that’s the point where a pregnancy test is worth taking.
Timing Depends on When You Took It
Plan B works by delivering a large dose of a synthetic hormone that can delay or prevent ovulation. That hormonal surge ripples through your cycle, and the effects look different depending on when you took it.
If you took Plan B early in your cycle, before ovulation, it can push ovulation back by several days. Since your period arrives roughly two weeks after ovulation, a delayed ovulation means a delayed period. In this scenario, a period that’s three to seven days late is common. If you took it closer to or after ovulation, your period is more likely to arrive on time or even a day or two early, because the hormone can shorten the second half of your cycle by a few days.
A study of 232 women found statistically significant changes in cycle length that varied based on whether emergency contraception was taken well before, close to, or well after expected ovulation. In other words, there’s no single answer to “how late will it be?” The timing is individual.
What Your Period Might Look Like
It’s not just the timing that can change. The period itself may feel different from what you’re used to. You might notice heavier bleeding, lighter flow, a longer duration, or some combination. Some people notice no change at all. Taking Plan B more than once in a single cycle increases the likelihood of noticeable flow changes.
Taking emergency contraception both before ovulation and later in the cycle has been shown to make a period last longer than usual. So if your period stretches an extra day or two beyond your norm, that’s a recognized side effect, not a red flag on its own.
Spotting Before Your Period Arrives
Some people experience light spotting or breakthrough bleeding in the days or weeks after taking Plan B, well before their actual period is due. This is not your period. It’s a side effect of the hormonal surge and can happen anywhere from a few days after taking the pill to several weeks later.
The key difference: spotting tends to be light, intermittent, and brownish or pinkish, while a true period follows its usual pattern of building flow. Once you get a recognizable period with your typical flow, that’s a reliable sign you’re not pregnant. Some people experience irregular bleeding for up to a month after taking Plan B, but it typically resolves on its own.
When a Delay Signals Something Else
The American College of Obstetricians and Gynecologists recommends clinical evaluation if your period is delayed by a week or more beyond when you expected it. A pregnancy test is reliable starting on the first day of a missed period. If you’re unsure when your period was due, take a test at least 21 days after the unprotected sex that prompted taking Plan B.
Heavy bleeding with intense pain is a separate concern. While heavier-than-normal flow is a common Plan B side effect, severe pain combined with heavy bleeding can signal an early pregnancy loss or another condition that warrants medical attention. Persistent irregular bleeding that continues beyond a month is also worth investigating, since at that point the cause is likely something other than Plan B.
Your Next Cycle Should Be Normal
The cycle disruption from Plan B is generally a one-time event. Research tracking women through the cycle after emergency contraception found that changes in cycle length, period duration, and flow were concentrated in the treatment cycle itself. By your second period after taking Plan B, your cycle should return to its baseline pattern. If it doesn’t, that’s worth a conversation with a healthcare provider, since something unrelated to the emergency contraception may be at play.
In the meantime, keep in mind that Plan B does not provide ongoing protection. You can become pregnant from sex that happens after you take the pill, even within the same cycle. If you had unprotected sex again after taking it, the timing calculations above won’t apply cleanly, and a pregnancy test becomes more important if your period doesn’t show up.