Reverse Due Date Calculator

Written by Great Medico

Updated September 24, 2026 •2 min read

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Pregnancy and fertility questions are full of dates, ranges, and probabilities—not certainties. Reverse Due Date Calculator narrows the question to the exact inputs shown and gives a practical estimate without pretending biology runs on a perfect calendar.

What this calculator does

Reverse Due Date Calculator uses Due date and Pregnancy length to calculate reverse due date. It is built around the exact fields displayed in this plugin rather than a generic version of the topic. Where supported, it also shows Estimated LMP and Likely intercourse window so the primary result is easier to place in context. Pregnancy and fertility dates, hormone trends, and risk scores are estimates built around biologic averages. Cycle variation, ultrasound findings, assisted-reproduction details, and the individual pregnancy can matter more than a single calculated number.

How to use it

Enter or select Due date and Pregnancy length. Use the units offered by the calculator and choose the option that matches the actual clinical or timing situation. Check date entries and laboratory units before relying on the output. For paired dates or serial measurements, keep them within the same menstrual cycle, pregnancy, or treatment episode as the calculator requires.

How the calculation works

The estimated conception date is calculated by subtracting 266 days from the entered due date. The estimated LMP is calculated by subtracting the selected pregnancy length, and the tool displays a likely intercourse window from five days before through one day after the estimated conception date.

Example

For example, from an EDD the calculator subtracts 266 days for estimated conception and uses the selected pregnancy length to work backward to an estimated LMP, then shows a seven-day intercourse window around conception.

How to interpret the result

Treat the output as estimated conception date based on the assumptions encoded in this calculator. It can help with planning or understanding a measurement, but pregnancy and reproductive biology vary enough that the calculated date, ratio, or probability should be seen as a range or estimate rather than a guarantee.

Limitations and notes

Working backward from a due date does not identify the exact day intercourse or fertilization occurred. Sperm can survive for several days, ovulation timing varies, and the EDD itself has uncertainty. Use the dates as an estimated window only.