How conception dating is calculated
Human gestation lasts an average of 266 days (38 weeks) from the moment of fertilization. In a typical 28-day cycle, conception occurs approximately 14 days after the first day of the last menstrual period.
Because sperm can survive within the female reproductive tract for up to 5 days while an unfertilized ovum lives for only 12 to 24 hours, intercourse taking place anywhere from 5 days prior to ovulation up to the day of ovulation can lead to fertilization.
Worked example: Reverse dating from due date
An expectant mother has an ultrasound-confirmed due date of August 15th.
Subtracting 266 days reveals an estimated conception date of November 22nd of the previous year.
The most probable intercourse window spans from November 19th through November 23rd.
Conception inputs
Calculation Reference
Select whether you know your estimated due date or last period.
Cycle Length
Used to adjust ovulation timing if calculating from LMP.
CALCULATOR QUESTIONS
Conception date & fertile window calculator questions
Can conception happen on the day of intercourse?
Yes, if intercourse occurs on the day of ovulation. However, conception frequently occurs 2 to 4 days after intercourse because sperm remain viable in the fallopian tubes awaiting egg release.
How accurate is conception date estimation?
Unless pregnancy occurred via IVF, conception date estimates carry an inherent ±3 to 5 day window of biological variation.