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Conception Calculator: Estimate When Conception Happened
Enter a due date or the first day of your last period and get the most likely conception date, the window in which intercourse could have led to it, and the matching start of pregnancy dating.
The arithmetic behind the estimate
Pregnancy is dated from the first day of the last period, but conception happens about two weeks later, when ovulation occurs in a typical 28-day cycle. Gestation from conception is therefore about 266 days, or 38 weeks. The calculator subtracts 266 days from a due date. If you start from the last period it adds cycle length − 14 days, because ovulation usually precedes the next period by about 14 days.
For a 35-day cycle starting on 1 March, ovulation is expected on day 21, which is 22 March, and not on day 14.
Why the result is a window
Sperm can survive in the reproductive tract for up to about five days, while the egg is viable for roughly a day after ovulation. So intercourse several days before ovulation can still result in conception on the day of ovulation. The window shown runs from five days before the estimated conception day to one day after it.
This matters if you are trying to identify a date of conception for personal reasons: more than one date of intercourse can fit the same due date, and the calculator cannot tell which one it was.
Accuracy and variation
Real pregnancies vary in length by several days either way even when the conception date is known, so working backwards from a due date carries that spread. A due date revised after a scan is based on measurement of the baby, which makes the back-calculated date less certain still. Ovulation timing also differs between cycles, even in the same person.
- Irregular cycles make the last-period method unreliable.
- Fertility treatment gives a documented date that supersedes any estimate.
- The ovulation calculator looks forward rather than backward, and the due date calculator converts dates in the other direction.
Using the result sensibly
Use the output to understand the timeline, to answer a clinic's questions about dating or for general curiosity. It is not proof of timing and is not suitable for any legal or evidential purpose. Questions about pregnancy, fertility or paternity belong with a clinician or the relevant professional.
Using the estimate sensibly
Dating from a due date or last period assumes a regular cycle, so treat the result as a window of several days rather than a single date. Ultrasound dating in early pregnancy is usually more accurate than cycle-based dating when the two disagree, and your clinician's dating is the one that counts. The calculator cannot establish paternity or confirm a pregnancy.
Worked example
A due date of 25 December 2026 is entered to find the most likely time of conception.
Inputs
| I know my | Due date |
|---|---|
| Date | 2026-12-25 |
Result
Most likely conception date. Intercourse from 29 March 2026 to 4 April 2026 could have led to it.
| Estimated conception date | Friday 3 April 2026 |
|---|---|
| Intercourse window | 29 March 2026 to 4 April 2026 |
| Equivalent first day of last period | 20 March 2026 |
| Estimated due date | 25 December 2026 |
- The estimate is a single most-likely day inside a window of about a week, not a certain date.
Medical disclaimer: This tool is for education and general planning. It is not medical advice and does not diagnose any condition. If you are pregnant, under 18, have an eating-disorder history or a medical condition, speak to a doctor or registered dietitian before changing your diet or exercise.
Frequently asked questions
How do I find my conception date from my due date?
Subtract 266 days, which is 38 weeks, from the due date. This puts conception about two weeks after the start of the last menstrual period in a 28-day cycle.
Can this tell me exactly which day I conceived?
No. It gives the most likely day and a window of about a week. Sperm survive for several days, so earlier intercourse can also result in conception.
Does my cycle length change the answer?
Yes, when you start from the last period. Ovulation follows about 14 days before the next period, so a 35-day cycle puts conception around day 21.
Why does my scan date differ from this estimate?
Ultrasound dating uses the size of the baby and may revise the due date. The back-calculated conception date moves with it.
Can I use this to plan a pregnancy?
For planning, the ovulation calculator is the better tool, because it projects fertile days forward from your cycle.
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References and sources
- Wilcox AJ, Weinberg CR, Baird DD (1995). Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine 333:1517-1521.
- American College of Obstetricians and Gynecologists (2017). Committee Opinion No. 700: Methods for Estimating the Due Date. Link