About the Pregnancy Conception Calculator
This calculator works backwards from a known due date to estimate when conception occurred. It is useful when dating has been established by ultrasound rather than by a remembered last period.
The formula
Conception ≈ due date − 266 daysThe 266-day figure is 280 days of gestational age minus the roughly 14 days between the last period and ovulation, giving time from conception to birth.
How to use this calculator
- 1Enter your Due Date Year. The field starts at
2027, which you can overwrite. - 2Enter your Due Date Month (1–12). The field starts at
6, which you can overwrite. - 3Enter your Due Date Day. The field starts at
1, which you can overwrite. - 4Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Due Date Year | 2027 |
| Due Date Month (1–12) | 6 |
| Due Date Day | 1 |
Result
Estimated Conception: September 8, 2026
Fertile Window Start: September 3, 2026
Last Menstrual Period: August 25, 2026
Due Date: June 1, 2027
Pregnancy Duration: 40 weeks (280 days)
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
Because a due date is usually itself an estimate, working backwards compounds the uncertainty. If the due date came from a first-trimester ultrasound it is accurate to about five days, so the conception estimate carries a similar margin. If it came from an LMP calculation with irregular cycles, the margin is considerably wider.
Conception occurs within a window rather than at a moment. Sperm survive up to five days and the egg 12 to 24 hours, so fertilisation could have happened across several days. This calculator identifies the most probable date, not a definite one.
Things worth knowing
- Due dates from first-trimester ultrasounds are the most reliable basis for working backwards.
- Gestational age counts from LMP, so it is about two weeks more than time since conception.
- The estimate spans several days. Treat it as a window, not a date.
- For paternity questions, DNA testing is the only reliable method.
- Later ultrasounds date less accurately, since fetal growth varies more as pregnancy progresses.
Frequently asked questions
How do I find my conception date from my due date?+
Subtract 266 days from the due date. That is 280 days of gestation minus the roughly two weeks between the last period and ovulation.
How accurate is this estimate?+
It inherits the uncertainty of the due date, so about five days if that came from a first-trimester scan and rather more if it came from an LMP calculation with irregular cycles.
Why is the conception date not exactly nine months before birth?+
Because pregnancy averages 266 days from conception, which is closer to eight and three-quarter months. The familiar nine months refers to the 280 days counted from the last period.
Can this establish paternity?+
No. The conception window covers several days and individual variation is considerable. DNA testing is the only definitive method.