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Reverse Due Date Calculator

Health

Estimate the last menstrual period (LMP) and conception timing from a known or estimated due date, working backward using the standard 280-day pregnancy length.

Written by · Reviewed by the thecalcu.com team · Last updated July 18, 2026

100

Estimated LMP (Days Ago)

180
Estimated Conception (Days Ago)
166

This calculator computes your Estimated LMP (Days Ago), Estimated Conception (Days Ago) from the values you enter.

Inputs
Days Until Due Date (Negative if Already Past)
Outputs
Estimated LMP (Days Ago)Estimated Conception (Days Ago)

Medical disclaimer: This calculator is for general information and educational use only. It is not a diagnosis and not a substitute for professional medical advice. Always consult a qualified healthcare provider about your health and before acting on any result.

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.

Why Use a Reverse Due Date Calculator?

Sometimes a due date is known before an LMP date is clearly established, and this calculator lets you work backward to estimate that starting reference point using standard pregnancy dating conventions.

Who Should Use This Calculator?

  • Anyone verifying pregnancy dating consistency between a due date and LMP.
  • Students of midwifery or nursing learning how pregnancy dating math works in both directions.
  • Healthcare educators demonstrating the relationship between LMP, conception, and due date.
  • Anyone researching historical pregnancy timeline reconstruction.

What Insights Does the Reverse Due Date Calculator Give You?

The Estimated LMP (Days Ago) shows how many days before today your last menstrual period would have started. The Estimated Conception (Days Ago) shows the same for approximate conception timing, roughly two weeks later than LMP.

How to use this Reverse Due Date calculator

  1. Enter the days until the due date (use a negative number if the due date has already passed).
  2. Read the Estimated LMP (Days Ago) and Estimated Conception (Days Ago) instantly.
Show formula & methodology ↓Hide formula & methodology ↑

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

What is a reverse due date calculation?

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.

How is estimated LMP calculated?

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.

How is estimated conception calculated?

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.

Why use 280 days for LMP but 266 for conception?

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.

Can I enter a negative number of days until due date?

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.

Is this calculator the same as the standard Pregnancy Due Date Calculator?

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.

Why would I need to work backward from a due date?

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.

Does this calculator account for ultrasound-adjusted due dates?

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.

How does this relate to the Gestational Age Calculator?

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.

Is this calculator a substitute for medical pregnancy dating?

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.
Also known as
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