Work backwards from a due date to the likely conception window.
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.
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.
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.
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.
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.