When did it all begin?
Enter your due date, or the actual birth date if the baby has already arrived, and see your estimated conception date instantly, along with the probable conception window, the fertile days of that cycle, the implantation window and when a pregnancy test first turned positive.
Your dates
Your estimated due date (or actual birth date for past pregnancies).
If the baby was already born, enter the birth date for a more precise estimate.
Conception typically happens about 266 days (38 weeks) before the due date. If you enter an actual birth date, it takes priority over the due date. Results update as you type.
What your dates say
From last period to a positive test
Key dates around conception
Day by day: what happened when
Every row is calculated backwards from the date you entered. Day 0 is the estimated conception day.
| Day | Approx. date | What typically happens |
|---|
Conception dating, explained
How Conception Date Is Estimated
Conception typically occurs about 266 days (38 weeks) before the due date. By subtracting 266 days from the due date, we arrive at an estimated conception date. Since sperm can survive up to 5 days in the reproductive tract and the egg is viable for about 24 hours after ovulation, the actual conception window spans approximately 5 days before to 1 day after the estimated date.
Conception Window Explained
The conception window represents the range of dates during which intercourse could have led to pregnancy. This calculator shows a window of plus or minus 5 days around the estimated conception date. The most likely conception occurred within 1-2 days of ovulation, but the wider window accounts for sperm survival time.
Common questions
Can I determine the exact day of conception?
It is very difficult to determine the exact day of conception because sperm can survive up to 5 days and fertilization may not occur until hours after intercourse. The calculator provides a probable window. The estimated date is the most likely, but conception could have occurred several days earlier.
Should I use the due date or birth date?
If the pregnancy is ongoing, use the due date estimated by your doctor. If the baby has already been born, using the actual birth date gives a more accurate estimate since it reflects when the baby was actually ready to be born rather than a predicted date.
How accurate is this estimate?
The estimate is accurate within about one week. Factors like irregular cycles, late or early ovulation, and the variability of gestational length (37-42 weeks) all affect precision. The conception window of plus or minus 5 days captures the most likely range.
This calculator gives an estimate, not a medical determination, and it is not legal or medical proof of paternity or of the exact day of conception. Sperm can survive up to 5 days, ovulation timing varies, and gestational length ranges from 37 to 42 weeks, so the true conception date can differ from the estimate by several days or more. For questions about pregnancy dating, ask your gynecologist or obstetrician, and for paternity questions rely on DNA testing, not calendar math.