What ovulation actually is

Once per cycle, one ovary releases a mature egg. That is ovulation.

It is a single event, not a phase. The egg then travels into the fallopian tube, where it can be fertilised for roughly 12 to 24 hours.

If nothing meets it in that time, the egg breaks down. Hormone levels fall, the uterine lining is shed, and a new cycle begins.

So the strictly fertile part of your cycle is less than a day long. Which raises an obvious question.

Why the fertile window is six days, not one

The window is longer than the egg’s lifespan because sperm get there first.

In fertile cervical mucus — the clear, stretchy kind that appears in the days before ovulation — sperm can survive up to five days. They can be present, viable and waiting well before an egg exists.

That is why the fertile window is best understood as:

five days before ovulation + ovulation day itself

The window opens before the event that defines it. Sex on any of those days can result in pregnancy, even though ovulation has not happened yet.

The six-day fertile windowA 28-day cycle showing the fertile window opening on day 9, five days before estimated ovulation on day 14.28-day cycle151015202528
  • Bleeding days
  • Fertile window
  • Estimated ovulation
  • Lower chance (not zero)
The window opens five days before ovulation because sperm can already be waiting.

What decides when you ovulate

The common assumption is that ovulation happens 14 days after your period starts. It is more accurate to say it happens about 14 days before your next period begins.

The phase after ovulation — the luteal phase — is relatively stable, usually 12 to 14 days. The phase before it is the variable one.

That means a useful estimate is your cycle length minus 14:

  • A 24-day cycle → ovulation around day 10
  • A 28-day cycle → ovulation around day 14
  • A 34-day cycle → ovulation around day 20

The signs your body gives you

Two signals are practical to track at home, and they do different jobs.

Cervical mucus — predicts ovulation

As oestrogen rises before ovulation, cervical mucus changes noticeably:

  • After your period: dry, or very little mucus
  • Approaching ovulation: creamy, then increasingly wet
  • Peak fertility: clear, slippery and stretchy, often compared to raw egg white
  • After ovulation: abruptly thicker, cloudier or dry again

The egg-white stage is the useful one. It tells you the window is open now, which a calendar cannot do.

Basal body temperature — confirms ovulation

After ovulation, progesterone raises your resting body temperature by roughly 0.2 to 0.5°C. Taken at the same time each morning before getting up, that shift is visible across a cycle.

The limitation is timing: temperature rises after ovulation. It confirms what already happened rather than warning you in advance.

Other signs worth noticing

  • Mittelschmerz — a one-sided lower abdominal ache around ovulation
  • Light spotting — brief and mid-cycle
  • Breast tenderness — usually after ovulation, as progesterone rises
  • Increased libido — often in the days just before ovulation

None of these are reliable enough to stand alone, but alongside mucus and temperature they add useful context.

When ovulation is not happening

Cycles without ovulation, called anovulatory cycles, become more common with irregular cycles, PCOS, thyroid conditions, very low body weight, high training loads, and in the years approaching menopause.

You may still bleed during an anovulatory cycle, which makes them easy to miss.

Signs worth investigating include cycles consistently shorter than 21 or longer than 35 days, no clear mucus pattern across several cycles, a temperature chart with no sustained rise, or difficulty conceiving after 12 months of trying — 6 months if you are over 35.