Pregnancy Conception Calculator
Estimate the likely conception date and last menstrual period from a known or expected due date.
About the Pregnancy Conception Calculator
This one runs the usual pregnancy dating math backwards. Instead of starting from your last period and calculating forward to a due date, you start from a due date — whether it's one your provider gave you or one you're just curious about — and this works back to an estimated conception date and an estimated start date for your last period.
Because the standard due-date convention (Naegele's rule) is built on a 280-day estimate from the last period and a 266-day estimate from conception, the numbers here are exactly as approximate as the due date you put in. If your due date came from an early ultrasound rather than LMP alone, the back-calculated LMP shown here may not match the actual first day of your last period.
People typically reach for this after they already have a due date in hand — from a provider, an app, or a pregnancy test result — and want to sanity-check roughly when conception happened, or figure out which date to answer when a form asks for their last period.
Nothing you enter here — the due date, the dates it calculates back to — is sent off your device or stored anywhere. It's just arithmetic that happens in your browser.
Frequently asked questions
How do you calculate conception date from a due date?
Subtract 266 days from the due date — that's the standard 280-day pregnancy length minus the roughly 14 days between the start of a last period and typical ovulation/conception.
Why would I need to know my last menstrual period if I already have a due date?
Prenatal intake forms and other pregnancy calculators often ask for LMP specifically, so working backward from a known due date is the easiest way to fill that in if you don't remember the actual date.
Is the conception date from this calculator exact?
No — it assumes ovulation happened exactly 14 days after the start of your last period, which is typical for a 28-day cycle but shifts earlier or later for shorter or longer cycles.
My due date came from an ultrasound — will the backward-calculated dates still be accurate?
They'll be less reliable in that case, since ultrasound dating is based on measuring the fetus directly rather than assuming a standard 28-day cycle, so the back-calculated LMP may not match your actual last period.
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