Emergency contraception & your cycle

Levonorgestrel emergency contraception works mainly by delaying or preventing ovulation — it suppresses the hormone surge (LH) that triggers the ovary to release an egg¹. The delay buys time for any sperm already present to die off before an egg is released. The current evidence is that it prevents or delays ovulation and does not work by preventing implantation¹.

It is most effective the sooner it is taken, and less effective the closer you already are to ovulation².

Because it can push ovulation later, it can shift the timing of everything that follows. Your next period may come earlier or later than usual — sometimes by a week or more — and can be lighter, heavier, or accompanied by light spotting in the days after you take it³.

In other words: a delayed or irregular cycle after emergency contraception is a normal, temporary effect of the medication — not a sign that anything is wrong, and usually not something the app got “wrong.”

Cycle Journal estimates ovulation and your next period from your temperatures and period dates. When emergency contraception delays ovulation, those estimates can look off for that one cycle. Noting the day you took it gives that cycle context — so a later-than-usual ovulation or period reads as “expected after EC” rather than a mystery. It also makes the pattern easy to see if you look back later.