Pregnancy Conception Calculator
Work backwards from a due date to the likely conception window.
About Pregnancy Conception Calculator
Working backwards from a due date: the LMP is 280 days before the EDD; ovulation typically occurs 14 days after the LMP; and the fertile window extends from about 3 days before to 2 days after ovulation. Conception (fertilisation) most likely occurred within this 5-day window.
Formulas
LMP from due date
LMP = Due Date − 280 days
Ovulation
Ovulation ≈ LMP + 14 days
Conception window
LMP + 11 days to LMP + 16 days
Worked example
With the default values below, Pregnancy Conception Calculator returns Most likely conception date of January 15, 2025. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Estimated due date
- 2025-10-08
Results
- Most likely conception date
- January 15, 2025
- Conception window start
- January 12, 2025
- Conception window end
- January 17, 2025
- Estimated LMP
- January 1, 2025
- Estimated ovulation
- January 15, 2025
Step by step
- 1LMP from EDD= January 1, 2025
- 2Ovulation (LMP + 14 days)= January 15, 2025
- 3Conception window= January 12, 2025 – January 17, 2025
Assumptions and limits
- The result is derived only from the single input listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- This is an estimate based on average cycle lengths and may not reflect your individual cycle. It should not be used for medical, legal or paternity purposes. Consult a healthcare provider for guidance.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
This is an estimate based on average cycle lengths and may not reflect your individual cycle. It should not be used for medical, legal or paternity purposes. Consult a healthcare provider for guidance.
Frequently Asked Questions
How accurate is the conception date estimate?
It assumes a regular 28-day cycle and ovulation exactly on day 14. For irregular cycles the actual date may shift significantly. An early ultrasound scan provides a more accurate gestational age than this calculation.
Why is the conception window 5 days wide?
Sperm can survive in the female reproductive tract for up to 5 days, and the egg remains viable for 12–24 hours after ovulation. The most likely conception dates are the 2 days before and the day of ovulation.
Can I use this to figure out paternity?
This calculator gives a statistical estimate based on population averages, not a clinical determination. For questions related to paternity, a DNA test is the only reliable method.