Pregnancy Conception Calculator
Work backwards from a due date to the likely conception window.
Inputs
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
LMP from EDD
EDD − 280 days
= January 1, 2025
Ovulation (LMP + 14 days)
= January 15, 2025
Conception window
Ovulation ± 2 days
= January 12, 2025 – January 17, 2025
How it works
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
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.