After taking Plan B, your period can be up to a week late compared to when you’d normally expect it. Most people get their period within two days to one week of their anticipated date, but a small percentage experience longer delays. The key threshold to remember: if your period hasn’t arrived within three weeks of taking Plan B, take a pregnancy test.
What the Clinical Data Shows
In the clinical trial used to approve Plan B, more than half of participants got their period within two days of when they expected it. About 31% experienced some change to their cycle, whether that meant an earlier period, a later one, or differences in flow. Only 4.5% of women had their period arrive more than seven days late.
So while a delay of a few days is common and not a red flag, a delay beyond a week is uncommon. It happens, but it puts you in a small minority of users, and it’s worth paying closer attention at that point.
Why Plan B Disrupts Your Cycle
Plan B delivers a large dose of a synthetic hormone that your body already produces in smaller amounts during your normal cycle. Its primary job is to delay or prevent ovulation, which is the release of an egg from your ovary. By flooding your system with this hormone all at once, Plan B essentially hits pause on your cycle’s normal progression.
The timing of when you take it matters. If you take Plan B early in your cycle, before ovulation, it can push ovulation back by several days, which in turn pushes your entire period back. If you take it later in your cycle, closer to when your period was already due, the delay tends to be shorter or nonexistent. Some people even get their period a few days early. The effect depends almost entirely on where you were in your cycle when you took the pill.
Early Period vs. Late Period
A late period gets more attention because it raises pregnancy concerns, but an early period is just as common after Plan B. Some people start bleeding within a few days of taking the pill, well before their expected date. This can be confusing because it might look like spotting rather than a full period, and it’s easy to mistake it for implantation bleeding or a side effect rather than an actual period.
Your first period after Plan B may also look different from what you’re used to. It can be heavier or lighter than normal, and you might notice more cramping or spotting between periods. These changes typically resolve by your next cycle. If your periods are still irregular two months later, that’s less likely to be a Plan B effect and more worth looking into.
The Three-Week Rule
Every major health organization uses the same benchmark: if three weeks pass after taking Plan B and you still haven’t gotten your period, take a pregnancy test. This isn’t because a three-week delay is common. It’s because by that point, a pregnancy test will be accurate enough to give you a reliable answer, and the delay is long enough to warrant checking.
A pregnancy test works by detecting a hormone your body produces after a fertilized egg implants in the uterus. That hormone needs time to build up to detectable levels. Testing too early, like a few days after taking Plan B, can give you a false negative simply because there hasn’t been enough time for the hormone to appear, even if conception did occur. Three weeks provides that buffer.
If you’re anxious and don’t want to wait the full three weeks, most home pregnancy tests are reasonably accurate about two weeks after unprotected sex. But the three-week mark after taking Plan B is when results are considered definitive.
What a Late Period Does and Doesn’t Mean
A period that’s three to five days late after Plan B is the most common scenario for people who experience a delay, and it almost always reflects the hormonal disruption from the pill itself, not a pregnancy. Plan B is 87% effective when taken within 72 hours of unprotected sex, so the odds are in your favor to begin with. A short delay on top of that is expected.
That said, Plan B is not 100% effective, and a late period can indicate pregnancy in rare cases. The factors that reduce Plan B’s effectiveness include taking it more than 72 hours after sex (it works best within the first 24 hours), body weight (some research suggests reduced effectiveness at higher body weights), and where you were in your cycle. If you had already ovulated before taking Plan B, the pill may not have been able to prevent fertilization.
A delay of more than seven days, while it only affects about 1 in 20 users, is the point where taking a pregnancy test becomes a reasonable step rather than something to wait on. You don’t need to wait the full three weeks if you’re already a week past your expected date and feeling anxious. A test at that point may not be perfectly conclusive, but it can offer some reassurance.
What to Expect From Your Next Few Cycles
Your first period after Plan B resets the clock. Most people return to a completely normal cycle within one to two months. The cycle in which you take Plan B is the one most likely to be irregular, whether that means it’s shorter than usual, longer, or accompanied by unexpected spotting. After that, your body’s hormonal rhythm typically corrects itself without any intervention.
If you start a regular form of birth control after using Plan B, your cycle may take a different shape depending on the method. The CDC recommends starting or resuming regular contraception after your next period following Plan B use, since the cycle in which you took emergency contraception may be unpredictable enough to make tracking difficult.