How this calculator works
This calculator estimates when conception likely occurred, working backward from either a due date or an actual birth date. It uses the standard 266-day convention: full-term pregnancy is commonly described as 266 days from conception to birth, which is the same as 280 days (40 weeks) from the last menstrual period, since ovulation and conception are conventionally assumed to occur about 14 days after the last period began.
Because ovulation timing varies by several days even in regular cycles, this calculator also shows a likely window of a few days around the point estimate rather than presenting a single date as certain. Treat the result as a general estimate - it is not a precise or verified determination of conception timing, and cannot be used as one.
The 266-day convention
Estimated conception date = due date (or birth date) − 266 daysLikely conception window = estimated date ± 3 daysCorresponding estimated LMP = estimated conception date − 14 days266 days is exactly 38 weeks, so the estimated conception date always falls on the same day of the week as the reference date used. This convention assumes a roughly average cycle and ovulation timing; it does not account for irregular cycles or medically assisted conception with a known fertilization date.
Worked example: due date October 8
- Subtract 266 days from October 8: this lands on January 15 of the same year.
- Likely conception window: January 15 ± 3 days, so January 12 through January 18.
- Corresponding estimated last menstrual period: January 15 − 14 days = January 1.
- Check: January 1 + 280 days (the standard due-date convention) = October 8, matching the original due date.
Frequently asked questions
How accurate is a conception date estimated this way?
It is a reasonable approximation for someone with regular cycles, typically useful to within about a week. It is not precise enough for legal or medical determinations that require an exact date - those situations generally rely on medical records such as IVF transfer dates or early ultrasound measurements.
Why use 266 days instead of 280 days?
280 days (40 weeks) is counted from the last menstrual period, which is roughly 2 weeks before ovulation and conception actually occur. 266 days is the same full-term length counted from conception itself, so the two figures describe the same pregnancy length from two different starting points.
Can I use the actual birth date instead of the due date?
Yes - select that mode. Keep in mind that most births do not happen exactly on the due date, so using the actual birth date shifts the estimate accordingly, which can be more or less accurate depending on how early or late the birth was relative to the original due date.
Why is there a window instead of a single exact date?
Ovulation does not happen on the same cycle day every month, even for people with regular periods. The few-day window reflects that normal variation rather than implying false precision.
How many weeks pregnant am I on the actual day of conception?
About 2 weeks, under the standard due-date convention - not 0 weeks. Pregnancy is conventionally counted from the last menstrual period (LMP) rather than from conception, and the estimated LMP falls 14 days before the estimated conception date. So on the day conception actually happens, you're already considered about 2 weeks (14 days) pregnant by that counting convention. Like the rest of this estimate, this is a convention, not a precise measurement of any individual pregnancy.