Conception Calculator
Enter the due date and cycle length to back-calculate the conception date and first day of last menstrual period.
Input Data
Estimated due date (EDD)
Average cycle length (default 28)
Results
Estimated date of conception
2026-01-15
First day of last menstrual period
2026-01-01
Due 2026-10-08 -> conception ~ 2026-01-15; LMP ~ 2026-01-01.
At a glance:Conception ~ due date - 266 days; LMP ~ due date - 280 - (cycle-28) days (reverse Naegele).
Formula
conception = due - 266 days.
lmp = due - 280 - (cycle - 28) days.
How to Use
- Pick your due date and average cycle length.
- Get the approximate conception date and last menstrual period.
Case Studies
Due 2026-10-08, cycle 28
Conception ~ 2026-01-15; LMP ~ 2026-01-01.
FAQ
Why conception = due - 266?
Pregnancy is about 280 days; conception is ~14 days after LMP, so it is about 266 days before the due date.
How does this differ from the Conception Date Calculator?
That tool goes forward from LMP; this one reverses from the due date.
Related Tools
References
Content reviewed by the Calculatorism editorial team. Results are for reference only; please refer to the relevant authorities for the official figures.