💞 Pregnancy Conception Calculator
If you already know a due date — from an early ultrasound, for example — you can estimate conception by counting back 266 days, the average time from ovulation to birth. The fertile window sits a few days around that date.
This is an estimate, not a paternity tool: sperm can survive several days, so conception can fall anywhere in roughly a one-week window.
How to use this calculator
Enter a known due date and this tool counts backwards to estimate when conception most likely happened, along with the fertile window around it.
- Pick your known Due date on the date field.
- Check the Estimated conception date in the results.
- Note the Likely fertile window, a few days on either side of that date.
- Compare with the Estimated LMP shown for your records.
Frequently asked questions
What does this conception calculator compute and what are the inputs?
It estimates the date conception most likely occurred, plus the fertile window surrounding it and the probable first day of the last period. The only input you need is a known due date, for example one from an early ultrasound.
What method does it use?
It subtracts 266 days from the due date, the average span from ovulation to birth in human gestation. The fertile window is then placed a few days either side of that date to reflect sperm survival and ovulation timing.
Can you show a worked example?
With a due date of 2027-04-05, counting back 266 days gives an estimated conception date of 2026-07-13, with a likely fertile window of roughly 2026-07-10 to 2026-07-15 and an estimated LMP of 2026-06-29.
Can this prove the exact date of conception?
No: conception can only be pinned down to about a one-week window because sperm can survive several days inside the body. Use it as a personal estimate, never as a paternity tool, and check anything important with your doctor.