Pregnancy Conception Calculator
Estimate your probable conception date window from an estimated due date, derived from the same formulas as the Due Date and Ovulation calculators.
Calculator Inputs
The 4-digit year of your estimated due date.
The month of your estimated due date (1-12).
The day of your estimated due date (1-31).
- Estimated Conception Date — Day
- 15
- Estimated Conception Date — Year
- 2026
- Estimated Conception Date — Month
- 1
- Conception Window Start — Year
- 2026
- Conception Window Start — Month
- 1
- Conception Window Start — Day
- 10
- Conception Window End — Year
- 2026
- Conception Window End — Month
- 1
- Conception Window End — Day
- 15
About these results
CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.
Health & Fitness Notice: Wellness and body metric calculations provide informational estimates based on generalized population formulas. They do not provide medical diagnosis, clinical advice, or treatment plans. Consult an appropriately qualified medical or healthcare professional for personal guidance.
How It's Calculated
Formula
This calculator estimates a probable conception date window by working backward from an estimated due date. The 266-day offset is not a separately invented number — it follows directly from this project's own verified Due Date formula (due date = LMP + 280 days) and Ovulation formula (ovulation = next period - 14 days): substituting one into the other shows conception falls 266 days before the due date (280 - 14 = 266), matching the standard obstetric estimate that conception typically occurs about two weeks after the last menstrual period. The window shown (5 days before the estimated date through the estimated date itself) mirrors the Ovulation Calculator's fertile-window convention, reflecting typical sperm viability. This is a PROBABLE ESTIMATE, not a confirmed conception date — it does not confirm pregnancy, confirm paternity, or guarantee exact fetal age.
Worked Examples
Estimated due date October 8, 2026
- Conception offset: due date - 266 days
- Oct 8, 2026 - 266 days = Jan 15, 2026
- Conception window: Jan 10 - Jan 15, 2026 (5 days before through the estimated date)
Frequently Asked Questions
Where does the 266-day offset come from?
It is derived from this project's own two verified formulas: the Due Date Calculator's 280-day LMP-to-due-date estimate, and the Ovulation Calculator's 14-day luteal-phase offset. Substituting one formula into the other shows the cycle-length terms cancel exactly, leaving conception 280 - 14 = 266 days before the due date.
Why is the result a window, not a single exact day?
Sperm can remain viable in the reproductive tract for up to about 5 days before ovulation, so the exact day intercourse led to conception cannot be pinpointed to a single calendar date from a due date alone. The window reflects that biological range, the same convention the Ovulation Calculator uses.
Does this confirm when I actually conceived?
No. This is a calendar-based estimate from your due date only, not a medical confirmation of the actual conception date or paternity.
How is this different from the Ovulation Calculator?
The Ovulation Calculator predicts a future fertile window forward from your last menstrual period. This calculator works backward from an already-estimated due date to a probable past conception window — the opposite direction, for someone who already has a due date estimate.