Fertile Window Back-Calculation
Estimate ovulation from cycle length, then cover the six fertile days
- Difficulty
- Easy
- Time to result
- ~weeks to results
- Steps
- 5
- Confidence
- 94%
Crawford defines the fertile window as the five days before ovulation plus the day of ovulation. Her back-calculation starts with a person's typical full cycle length, measured from the first day of bleeding to the day before the next period. She describes the post-ovulation corpus luteum phase as approximately 14 days, so subtracting 14 from cycle length gives a rough ovulation day: day 14 for a 28-day cycle and day 21 for a 35-day cycle. The method then marks the prior five days because sperm can survive in the reproductive tract before the egg arrives. Crawford also offers a simpler operational rule: if cycles are regular, intercourse every day or every other day can cover the window without intensive tracking. Irregular or absent cycles require medical evaluation rather than more calendar arithmetic.
Origin
Extracted from The Diary of a CEO
Core principles
- 01Cycle length varies between people
- 02The post-ovulation phase is approximately two weeks
- 03Sperm can be present before ovulation
- 04Regular intercourse can reduce dependence on perfect tracking
How to run it
- 1
Measure complete cycles
Count from the first day of one bleed to the day before the next bleed. Use several cycles to identify the person's typical length and variability.
Pro tip Track actual bleeding dates rather than relying only on an app prediction.
Watch out A population-average 28-day cycle is not a personal default.
- 2
Check regularity first
Confirm that the period arrives within a predictable range. If timing is irregular or periods are absent, stop treating the calendar as reliable.
Watch out Crawford says irregular or absent periods can signal an ovulation or hormonal problem that warrants medical care.
- 3
Estimate ovulation
Subtract 14 from the typical cycle length to estimate the ovulation day. Treat this as an approximation rather than a precise measurement.
Pro tip Recalculate when the usual cycle length changes.
Watch out Individual luteal phases and cycles can vary.
- 4
Open the six-day window
Mark the estimated ovulation day and the five days before it. This reflects Crawford's explanation that sperm can survive before the egg is released.
Pro tip Beginning before ovulation avoids depending on a single perfectly timed attempt.
- 5
Cover the window consistently
Have intercourse every day or every other day through the window if that is workable. Regular sex two or three times a week may also create useful coverage without making one day carry all the pressure.
Pro tip Choose a sustainable cadence that does not turn intimacy into a single high-stakes appointment.
Watch out Do not deliberately abstain to save sperm for ovulation.
In the wild
For a regular 28-day cycle, subtracting 14 estimates ovulation around cycle day 14. The estimated fertile window therefore begins around day 9 and includes day 14, with intercourse spread through that period rather than reserved for one day.
→ The couple covers the days before the egg is released as well as the estimated ovulation day.
For a regular 35-day cycle, subtracting 14 estimates ovulation around cycle day 21. The likely window shifts later than a generic app might assume, beginning around day 16 and including day 21.
→ Timing reflects the person's longer cycle rather than the 28-day convention.
Common mistakes
Using day 14 for everyone
Crawford's examples show that estimated ovulation changes with total cycle length.
Saving sperm for one day
Deliberate abstinence can reduce coverage and may increase the proportion of dead sperm after long intervals.
Is it for you?
Best for
It is best for couples trying to conceive when menstrual cycles are regular and intercourse is possible throughout the estimated window.
Not ideal for
It is not ideal for irregular or absent cycles, diagnosed fertility conditions, or anyone needing individualized medical assessment.
From the transcript
“the fertile window is going to be the 5 days before and then the day you ovulate”
“if your Cycles are 35 days though it's very different”
From the episode
The Pregnancy Doctor: Pregnancy Halves Every Year After 32! If You Want 2+ Children, You Need To Know This! If You Experience This Pain, Go See A Doctor!