Plan B (levonorgestrel 1.5 mg) typically delays ovulation for about 5–7 days when taken before your body’s LH surge (the hormone spike that triggers egg release).

How long Plan B can delay ovulation

  • Plan B mainly works by postponing ovulation , not by affecting an egg that has already been released.
  • Studies and clinical guidance suggest ovulation is usually delayed around 5–7 days , which roughly matches how long sperm can survive in the reproductive tract (up to 5 days).
  • The idea is to “stall” ovulation long enough for any sperm present to die off before an egg appears.

What affects how long it’s delayed?

How long Plan B delays ovulation isn’t identical for everyone.

  • Timing in your cycle: It works best when taken before the LH surge; close to or during ovulation it may not delay ovulation at all, so pregnancy protection drops a lot.
  • Body factors: Weight/BMI, metabolism, and some medications (especially those affecting liver enzymes) can make levonorgestrel less effective at delaying ovulation.
  • Dose timing: The sooner after unprotected sex you take it (ideally within 72 hours), the better its chance to delay ovulation in time.

Does Plan B still work if you’re ovulating?

  • If ovulation is already happening or just happened , Plan B is unlikely to work , because it mainly prevents or delays egg release and does not block fertilization or implantation.
  • That’s why many medical sources say it’s not reliable during or after ovulation , even though it’s still safe to take if you’re unsure where you are in your cycle.

How long can it affect your cycle?

  • Because Plan B pushes ovulation later, your next period may be up to about a week early or late , and flow can be lighter, heavier, or more spotty than usual.
  • Most people’s cycles go back to their usual pattern by the following month.

Quick example scenario

Imagine your typical ovulation day is Day 14 of your cycle:

  • You have unprotected sex on Day 11 and take Plan B a few hours later.
  • Plan B may delay ovulation from Day 14 to around Day 19, so by the time your egg is released, sperm from Day 11 are no longer viable, lowering the chance of pregnancy.

Important notes

  • Plan B is emergency contraception, not meant as a regular birth control method.
  • If your period is more than a week late after taking it, most experts recommend taking a pregnancy test and/or talking to a healthcare professional.
  • If you’re close to ovulation or after it, a copper IUD or a prescription emergency pill like ulipristal acetate may be more effective options; these require medical consultation.

Information gathered from public forums or data available on the internet and portrayed here.