Conception Date Calculator
Work backward from a due date, an actual birth date, or the first day of the last menstrual period to estimate when conception most likely occurred — plus the 6-day fertile window around it.
Estimated conception
Note: Conception is estimated, not measured. Sperm can fertilize an egg up to five days after intercourse, so the fertile window covers roughly six days.
How conception is estimated
For a typical 28-day menstrual cycle, ovulation happens around day 14, and conception (fertilization of the egg) follows within roughly 24 hours. Pregnancy is dated from the first day of the last menstrual period (LMP), which is about two weeks before conception. That's why a "40-week pregnancy" is really 38 weeks of fetal growth plus the two pre-conception weeks counted from the LMP.
The math
- From due date: subtract 266 days (38 weeks) → estimated conception day.
- From birth date: subtract 266 days → estimated conception day.
- From LMP: add 14 days, adjusted by your cycle length offset.
Understanding the fertile window
Conception itself happens on a specific day, but the fertile window is the six-day stretch ending on the day of ovulation. Sperm can survive in the female reproductive tract for up to five days, while the egg is only viable for about 12 to 24 hours after ovulation.
When this calculator is useful
- When you don't remember the date of your last period.
- When tracking back to compare against ultrasound dating.
- When understanding the difference between gestational age and embryonic age.
Medical disclaimer: This calculator is for informational purposes only. The exact day of conception cannot be determined from outside data alone. Use it for general understanding, not for medical, legal or paternity decisions.