Last updated October 5, 2026 · Report an error
While most pregnancy calculators work forward from your last period to estimate a due date, sometimes you already know the due date and want to estimate when conception likely occurred.
This calculator works backward from a due date to estimate the likely conception date and window.
How it works
Enter your expected due date. The calculator subtracts 266 days — the average length of a full-term pregnancy measured from conception — to estimate the conception date, and shows a ±5 day window to reflect normal fertility timing variation.
- Enter expected due date.
- Click Calculate to see your results.
Examples
Due date December 25
A due date of December 25 corresponds to an estimated conception date of about April 3 of the same year, with a likely window from March 29 to April 8.
Who should use it
- General curiosity about when conception likely occurred during a pregnancy.
- Cross-checking timing against other known dates or events.
Industry applications
- Personal pregnancy tracking
- General reproductive health education
Advantages
- Works directly from a due date, useful when that's the only known reference point.
- Provides a realistic window rather than a falsely precise single date.
Limitations
- Based on an average 266-day pregnancy length — individual pregnancies vary in actual length.
Common mistakes to avoid
- Treating the estimated conception date as medically precise rather than a statistical estimate with natural variation.
- Confusing this backward-looking estimate with the Ovulation Calculator, which works forward from your last period.
Best practices
- Use the full window (not just the single estimated date) when the timing matters for any practical decision.
Tips
- If you know your actual ovulation or last period date, the Ovulation Calculator or Due Date Calculator will generally give a more precise, forward-looking estimate than working backward from a due date.