How to Calculate Fertility Window

What is Fertility Window?

The fertility window is the 6-day period when conception is possible — 5 days before ovulation and ovulation day itself. Ovulation typically occurs 14 days before the next period.

Formula

Ovulation typically day 14 of 28-day cycle (range 12–16). Fertile window: day 12–16 (5 days pre-ovulation, day of ovulation). Sperm viable 5 days; egg 12–24 hours.

CycleLength: Menstrual cycle length (Days (typical 28, range 21–35))
OvulationDay: Expected ovulation (Days from cycle start)
FertileWindow: Peak fertility window (Days (5–6 day window))

Step-by-Step Guide

1Ovulation ≈ Cycle length − 14 days from LMP
2Fertile window = Ovulation − 5 days to Ovulation day
3Sperm survive 3–5 days; egg survives ~12–24 hours
4OPK tests and basal body temperature confirm ovulation

Worked Examples

Input

30-day cycle, LMP 1 January

Result

Ovulation ≈ Jan 16; Fertile window: Jan 11–16

Frequently Asked Questions

How do I predict ovulation?

Calendar method: ovulation = cycle length − 14 days (average). LH surge kits detect surge 24–36 hrs before ovulation (most accurate). Basal body temperature rise (0.5°C) confirms ovulation post-hoc.

Does cycle length variation matter?

Yes. 28-day cycles: simple prediction. 21–35 days normal but unpredictable. Track 3 months minimum to establish pattern. Apps (Clue, Flo) help predict best.

What decreases conception chances?

Irregular cycles, anovulation (no ovulation), poor egg/sperm quality, timing (intercourse every 2–3 days throughout cycle > precise timing), age (female fertility drops 35+).

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