Pregnancy Conception Calculator
Use this pregnancy conception calculator to work backward from an estimated due date to an approximate conception date and fertile window.
Conception estimates are approximate because ovulation timing varies.
How it's calculated
Estimated conception = due date - 266 days
Pregnancy due dates are often estimated as 280 days from the last menstrual period, or about 266 days from conception. Real ovulation timing can vary.
What your result means
| Category | Range | What it means |
|---|---|---|
| Estimated conception | Due date - 266 days | Approximate midpoint estimate. |
| Fertile window | About 5 days before ovulation to 1 day after | A broader window because sperm can survive for several days. |
| Estimated LMP | Due date - 280 days | Back-calculated last menstrual period estimate. |
Why conception dates are estimates
Even with a due date, conception can only be estimated because ovulation and implantation timing vary.
Fertile window basics
The fertile window usually includes the five days before ovulation and the day after ovulation.
Ultrasound and clinical dating
Early ultrasound can refine pregnancy dating more accurately than calendar math alone.
Frequently asked questions
Can I know the exact conception date?
Usually no. Calendar methods estimate a window, not an exact moment.
How far before a due date is conception?
A common estimate is about 266 days before the due date.
What is the fertile window?
It is the window around ovulation when pregnancy is possible, often estimated as five days before ovulation through about one day after.
Can ultrasound change the estimate?
Yes. Clinical dating may adjust calendar estimates, especially when cycle timing is uncertain.