Conception Date Calculator
Enter your due date to estimate when conception occurred.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator works backwards from a due date or birth date to estimate when conception occurred. Here is how to use it:
- Enter your due date. If your baby has already been born, you can enter the birth date instead. The calculation works the same way.
- Read your results. The calculator shows the estimated conception date (due date minus 266 days), the estimated last menstrual period date (due date minus 280 days), and the likely fertile window during which conception occurred.
These dates are estimates based on standard 40-week pregnancy duration and a 14-day ovulation window. Individual conception dates can vary by several days or weeks, especially if ovulation was early or late.
About the Conception Date Calculator
Standard pregnancy is counted as 280 days (40 weeks) from the last menstrual period. Since ovulation and conception typically occur around day 14 of the cycle, conception is approximately 266 days (38 weeks) before the due date. The fertile window, when intercourse could result in the pregnancy, spans from 5 days before ovulation through ovulation day, since sperm can survive for up to 5 days.
Due dates themselves have a margin of error of several weeks, so the conceived-on date carries the same uncertainty. If you need legal documentation of conception for custody or paternity purposes, only a court-ordered DNA paternity test can provide certainty. This calculator is for informational use only.
Frequently Asked Questions
How do I figure out the exact date I conceived?
The exact date of conception cannot be determined from a due date alone. Conception could have occurred any day within the 6-day fertile window (5 days before ovulation through ovulation day). Ovulation prediction kits and basal body temperature records from around the time of conception can narrow the window. An early ultrasound can estimate gestational age to within a few days, but it still reflects a range rather than a single date.
Can I use a birth date instead of a due date?
Yes. You can enter the baby's birth date and get an estimated conception date. Keep in mind that babies are born anywhere from 37 to 42 weeks, so a baby born at 37 weeks was conceived about 3 weeks earlier than the due date formula implies. For a term birth (39-40 weeks), the estimate will be accurate. For significantly preterm or post-term births, adjust by the number of weeks early or late.
How far back does the fertile window extend before ovulation?
Sperm can survive in the female reproductive tract for up to 5 days, so intercourse up to 5 days before ovulation can result in conception. The egg itself survives only 12-24 hours after ovulation. This means the fertile window extends 5 days before ovulation through the day of ovulation. Intercourse after ovulation day has essentially zero chance of resulting in pregnancy that cycle.
What if my due date was changed by an ultrasound?
Use the ultrasound-adjusted due date for the most accurate conception estimate. Ultrasound dating is more reliable than LMP dating, especially if your cycles are irregular. Enter the revised due date from the ultrasound and the calculator will work backwards from that date. The resulting conception date will reflect the revised gestational age rather than the LMP-based estimate.