When Are You Most Fertile? The Fertile Window

Why the window opens before ovulation

Two survival times set the whole thing:

up to 5 days

sperm survival

12–24 hours

egg viability

Sperm present before the egg is released can wait for it. An egg released and not met within a day is gone. That asymmetry is why the fertile window sits almost entirely before ovulation, and closes almost immediately after.

Relative chance of conception — 28-day cycle

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Cycle day. Peak is the two days before ovulation on day 14 — the day after drops away sharply.

Finding your own ovulation day

ovulation day = cycle length − 14
fertile window = ovulation day − 5 → ovulation day

The luteal phase — ovulation to the next period — is relatively fixed at 12 to 16 days. Nearly all the variation between people and between months sits in the first half of the cycle, which is why counting backward from the next period is more reliable than counting forward from the last one.

Cycle length	Ovulation ≈	Fertile window
26 days	Day 12	Days 7–12
28 days	Day 14	Days 9–14
30 days	Day 16	Days 11–16
32 days	Day 18	Days 13–18
35 days	Day 21	Days 16–21

Day 1 is the first day of full bleeding. On a 32-day cycle, someone targeting "day 14" would be finishing four days before their window even opened.

Map your fertile window across the next three cycles.Ovulation calculator →

Signs that predict versus signs that confirm

Sign	Timing	Use
Cervical mucus turns clear and stretchy	Days before	Predicts
LH surge on an ovulation test	24–36 h before	Predicts
Mild one-sided ache (mittelschmerz)	Around ovulation	Suggestive
Basal body temperature rise	After	Confirms only

Why BBT charting frustrates people: the temperature rise happens after the egg is released, so by the time the chart shows it the window has closed. It is excellent for learning the shape of your cycles over several months — and useless for timing this one.

What shifts the window

Cycle variation. Most people's cycles vary by a few days month to month, so treat any predicted date as a centre point rather than a fixture.
Stress, illness, travel and shift work commonly delay ovulation, which lengthens that cycle.
Coming off hormonal contraception can leave cycles irregular for a few months.
Irregular cycles — consistently shorter than 21 days, longer than 35, or unpredictable — make calendar prediction unreliable, and are worth raising with a clinician.

This is general information about how cycles work, not medical advice, and calendar methods are not a reliable form of contraception. If you have been trying to conceive for 12 months (or 6 months if over 35), speak to a doctor.

Related: predicting your next period from the same cycle data, and working out when conception happened after a positive test.

Run your own numbers

Ovulation Period Conception

FAQ

When is the fertile window?

The roughly six days ending on ovulation day: the five days before, plus ovulation itself. It works this way because sperm can survive up to five days in the reproductive tract, while the egg is only viable for about 12 to 24 hours after release.

How do I know when I ovulate?

Subtract 14 from your cycle length. The luteal phase — ovulation to next period — is relatively constant at 12 to 16 days, while the first half varies. On a 30-day cycle that puts ovulation near day 16, not day 14.

Is day 14 always ovulation day?

Only on a textbook 28-day cycle. Since cycle length varies from person to person and month to month, day 14 is a reasonable average and a poor individual prediction. Most of the variation sits in the phase before ovulation.

What are the signs of ovulation?

Cervical mucus becoming clear, stretchy and egg-white in texture is the most useful advance sign. An LH surge on an ovulation test appears 24 to 36 hours beforehand. A basal body temperature rise confirms ovulation has already happened, so it maps past cycles rather than predicting the current one.

Can you ovulate more than once a cycle?

Not more than once per cycle in the sense of separate events days apart. Multiple eggs can be released within roughly 24 hours of each other, which is how non-identical twins occur, but a second independent ovulation later in the same cycle does not happen.

Sources

Primary references used for the figures and rules on this page.

Fertility Awareness-Based Methods of Family Planning — ACOG
Preconception Care — NIH — NICHD