How to Estimate Your Conception Date

Starting from your last period

Pregnancy dating traditionally counts from the first day of the last menstrual period (LMP), because that date is easier to remember than the moment of conception. In a typical 28-day cycle, ovulation happens around day 14, and conception can occur when an egg is fertilized in the day or two after ovulation. That places likely conception roughly two weeks after the LMP.

This is why doctors say a pregnancy is a certain number of weeks even though conception happened later: the LMP-based clock starts about two weeks before conception. A conception calculator does this arithmetic for you, adjusting for your cycle length so you do not have to count days by hand.

Working back from a due date

If you already have an estimated due date — often from an early ultrasound — you can work backwards. Pregnancy is counted as about 40 weeks from the LMP, but only about 38 weeks (266 days) from actual conception. So subtracting 266 days from the due date gives an estimated conception date.

Early ultrasound dating is usually more accurate than the LMP method, especially for irregular cycles, so if a scan has given you a due date it is a good basis for this calculation. The calculator lets you enter either the LMP or the due date and returns the matching estimate.

Why it is an estimate

Conception dates are estimates because ovulation does not always fall on day 14, cycles vary in length, and sperm can survive for several days, so fertilization may not coincide with intercourse. Even careful tracking usually points to a window of a few days rather than a single moment.

For medical purposes, a healthcare provider establishes the official dating, usually confirmed by ultrasound. Use an estimated conception date for personal interest and planning, and rely on your provider for the dates that guide your care.

Frequently asked questions

Is the conception date the same as the ovulation date?

They are very close but not identical. Fertilization occurs after ovulation, and because sperm can survive for several days, conception may happen a day or two after intercourse. Most estimates treat ovulation day as a close stand-in for conception.

What if my cycle is not 28 days?

Then ovulation likely falls earlier or later than day 14. A longer cycle tends to push ovulation later and a shorter cycle earlier. Entering your own cycle length into the calculator adjusts the estimate accordingly.

How accurate is a conception date estimate?

It is a best estimate with a margin of several days, not an exact date. Early ultrasound dating is generally the most accurate reference, and a healthcare provider can give the most reliable figure for your pregnancy.

Can I use my due date if I don't remember my last period?

Yes. If you have a due date from a scan or provider, counting back about 266 days gives an estimated conception date without needing the last period, which is helpful when the LMP is unknown or cycles are irregular.