Conception Calculator
Estimated Conception Date Range
Enter a known due date or birth date to estimate the likely window when conception occurred.
Parameters
Estimated Conception Window
Understanding Conception Date Estimation
This calculator works backward from a known due date or actual birth date to estimate when conception likely occurred — the reverse of the standard due-date calculation.
Reverse Naegele's Rule
Since a due date is typically LMP + 280 days, and conception is ~14 days after LMP, conception is estimated as due date - 266 days.
Using an Actual Birth Date
If the baby was born on or near the due date, actual birth date can be used as a substitute for due date in this estimate.
A Window, Not a Single Day
Because sperm can survive several days and ovulation timing varies, conception is best understood as a several-day window rather than one exact date.
Not a Legal or Paternity Tool
This estimate is for general informational purposes — it cannot determine an exact conception date for legal or paternity purposes.
Important Limitations
- Uses population averages: Actual gestation length varies naturally, so the estimate is a probable range, not a certainty.
- Birth date is an imperfect substitute: If the birth happened well before or after the original due date, this estimate becomes less reliable.
- Not for legal determinations: Legal paternity or conception-date questions require clinical/genetic evidence, not calculator estimates.
Key Takeaways
- This reverses the standard due-date formula. Conception ≈ due date - 266 days.
- Results are a window, not a single point. Expect a range of several days around the calculated estimate.
- Actual birth date is a reasonable substitute for a due date if the pregnancy went close to full term.
- Consult a professional. Conception-date estimates are approximate ranges, not legal or medical proof of a specific date — confirm gestational details with your healthcare provider.