Reverse Due Date Calculator

Use the reverse due date calculator to start with a due date and estimate the pregnancy timeline behind it. The result shows the estimated last menstrual period, estimated conception date, first and second trimester endpoints, third trimester start, development milestones, and appointment timing.

How to use this calculator

Enter the due date shown by your clinician, app, or planning scenario. The calculator immediately works backward using standard gestational dating. If you want to estimate from a period date or ultrasound instead, use the gestational age calculator or the pregnancy conception calculator.

How it works

Pregnancy due dates are commonly estimated as 40 weeks after the first day of the last menstrual period.

$estimated\ LMP = due\ date - 280\ days$

$estimated\ conception = due\ date - 266\ days$

Milestones are placed by counting pregnancy weeks forward from the estimated LMP or backward from the due date.

Worked example

For a due date of October 8, the calculator estimates an LMP around January 1 and conception around January 15. The first trimester ends about 13 weeks after the LMP, and the third trimester begins around week 27.

Reading the timeline

The dates are planning estimates, not guarantees. Actual conception can vary, especially when cycles are irregular or the due date was revised. For calendar intervals unrelated to pregnancy, the age calculator or time duration calculator may be more appropriate.