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Conception Date Calculator

Estimate the likely conception window from a due date or birth date.

Likely conception window

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What this calculator does

This conception date calculator works backward from a due date or a baby's birth date to estimate the likely conception window, based on a standard 266-day gestation period from conception to birth.

Worked example

For a due date of June 1, 2027, the likely conception window is approximatelySeptember 5–10, 2026, centered on an estimate of September 8, 2026.

Common mistakes

  • Treating the estimate as a single exact day. Conception can only be pinpointed to a multi-day fertile window, not one precise date.
  • Using the last-period date instead of the due date. This calculator expects a due date or birth date — for a last-period-based estimate, use the pregnancy due date or ovulation calculator instead.
  • Assuming full-term timing for an early or late birth. Premature or overdue babies shift the true conception date earlier or later than this average-based estimate.

Frequently asked questions

Why is a conception date calculator a window instead of a single exact date?+

Conception can only happen within a fertile window of a few days around ovulation, since an egg survives about 24 hours and sperm can survive up to 5 days. This calculator shows that likely window rather than a false single-day precision.

How does this work from a due date?+

A standard due date assumes a 280-day (40-week) pregnancy from the last menstrual period, with ovulation and likely conception occurring around day 14 of that cycle — so we estimate conception as roughly 266 days before the due date.

How does this work from a birth date?+

The same logic applies using the actual birth date instead of the estimated due date, assuming a typical full-term pregnancy length.

Is this accurate for premature or late births?+

No — this calculator assumes an average, full-term pregnancy length. Premature or overdue births shift the actual conception date earlier or later than this estimate.

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