Reverse Due Date Calculator
HealthEstimate the last menstrual period (LMP) and conception timing from a known or estimated due date, working backward using the standard 280-day pregnancy length.
100
Estimated LMP (Days Ago)
180
Estimated Conception (Days Ago)
166
What is a Reverse Due Date?
The Reverse Due Date Calculator estimates your last menstrual period (LMP) and approximate conception timing by working backward from a known or estimated due date, using the standard 280-day (LMP) and 266-day (conception) pregnancy length references.
For the forward version of this calculation, see the Pregnancy Due Date Calculator.
How to use this Reverse Due Date calculator
- Enter the days until the due date (use a negative number if the due date has already passed).
- Read the Estimated LMP (Days Ago) and Estimated Conception (Days Ago) instantly.
Formula & Methodology
Estimated LMP (Days Ago) = 280 − Days Until Due Date Estimated Conception (Days Ago) = 266 − Days Until Due Date Worked example — a due date 100 days from now: Estimated LMP = 280 − 100 = 180 days ago Estimated Conception = 266 − 100 = 166 days ago
Frequently Asked Questions
A reverse due date calculation works backward from a known or estimated due date to estimate the last menstrual period (LMP) date and approximate conception timing, using the standard pregnancy length references.
The calculator subtracts your entered days until due date from 280 (the standard number of days from LMP to due date), giving how many days ago your LMP would have been.
The calculator subtracts your entered days until due date from 266 (the standard number of days from conception to due date), since conception typically occurs about 2 weeks after LMP.
Standard pregnancy dating counts from LMP, which occurs before ovulation and conception — the 14-day gap between the two figures (280 vs 266) reflects the typical time between the start of a period and ovulation.
Yes — a negative number represents a due date that has already passed, which is useful for working backward on a historical pregnancy or verifying LMP dates after the fact.
No — the [Pregnancy Due Date Calculator](/pregnancy-due-date-calculator/) projects forward from a known LMP to estimate a due date, while this calculator works in the opposite direction, starting from a due date to estimate LMP and conception.
This can be useful for verifying dating consistency, understanding historical pregnancy timelines, or cross-checking an estimate when the due date is known but the LMP date is uncertain or was recorded differently.
This calculator uses the standard 280/266-day reference figures regardless of how the due date was originally determined; if your due date was adjusted based on an ultrasound rather than LMP, the reverse-calculated LMP may not precisely match your actual LMP date.
The [Gestational Age Calculator](/gestational-age-calculator/) calculates current gestational age forward from a known LMP, while this calculator works backward from a due date — the two use complementary approaches to the same underlying pregnancy dating math.
No — this is a general reference calculator using standard day-count conventions. Official pregnancy dating, especially when precision matters, should come from your healthcare provider's clinical assessment and any ultrasound dating performed.
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