Reverse Due Date Calculator
Enter a due date to find the estimated last menstrual period (LMP) and conception window it implies, using Naegele's rule.
Frequently Asked Questions
How do you calculate LMP from a due date?
By subtracting 280 days (40 weeks) from the due date — the reverse of Naegele's rule, the standard rule of thumb used to estimate a due date from the first day of the last menstrual period in the first place.
What is Naegele's rule?
A rule of thumb from 19th-century obstetrics: due date = LMP + 280 days (the classic mnemonic is 1 year, minus 3 months, plus 7 days, which approximates but does not always match the 280-day count, since calendar months are not all the same length). It assumes a textbook 28-day cycle with ovulation on day 14, so it's a useful estimate, not a precise calculation for every cycle length.
How is the conception date estimated?
As 14 days after the LMP, which is the typical timing of ovulation in a 28-day cycle. Because ovulation timing varies, this page also shows a wider window (LMP + 10 to 19 days) rather than a single date.
Is this accurate enough to plan around?
It's a standard estimate, not a diagnosis. Actual due dates are normally confirmed or adjusted by a clinician, often using an early ultrasound, which can be more accurate than LMP-based dating alone, especially for irregular cycles.